[RFC v3] iio: input-bridge: optionally bridge iio acceleometers to create a /dev/input interface
Some user spaces (e.g. some Android devices) use /dev/input/event* for handling the 3D position of the device with respect to the center of gravity (earth). This can be used for gaming input, auto-rotation of screens etc.
This interface should be the standard for such use cases because it is an abstraction of how orientation data is acquired from sensor chips. Sensor chips may be connected through different interfaces and in different positions. They may also have different parameters. And, if a chip is replaced by a different one, the values reported by the device position interface should remain the same, provided the device tree reflects the changed chip.
This did initially lead to input accelerometer drivers like drivers/input/misc/bma150.c or drivers/misc/lis3lv02d/
But nowadays, new accelerometer chips mostly get iio drivers and rarely input drivers.
Therefore we need something like a protocol stack which bridges raw data and input devices. It can be seen as a similar layering like TCP/IP vs. bare Ethernet. Or keyboard input events vs. raw gpio or raw USB access.
This patch bridges the gap between raw iio data and the input device abstraction so that accelerometer measurements can additionally be presented as X/Y/Z accelerometer channels (INPUT_PROP_ACCELEROMETER) through /dev/input/event*.
There are no special requirements or changes needed for an iio driver.
There is no need to define a mapping (e.g. in device tree).
This driver simply collects the first 3 accelerometer channels as X, Y and Z. If only 1 or 2 channels are available, they are used for X and Y only. Additional channels are ignored.
Scaling is done automatically so that 1g is represented by value 256 and range is assumed to be -511 .. +511 which gives a reasonable precision as an input device.
If a mount-matrix is provided by the iio driver, it is also taken into account so that the input event automatically gets the correct orientation with respect to the device.
If this extension is not configured into the kernel it takes no resources (except source code).
If it is configured, but there is no accelerometer, there is only a tiny penalty for scanning for accelerometer channels once during probe of each iio device.
If it runs, the driver polls the device(s) once every 100 ms. A mode where the iio device defines the update rate is not implemented and for further study.
If there is no user-space client, polling is not running.
The driver is capable to handle multiple iio accelerometers and they are presented by unique /dev/input/event* files. The iio chip name is used to define the input device name so that it can be identified (e.g. by udev rules or evtest).
Here is some example what you can expect from the driver (device: arch/arm/boot/dts/omap3-gta04a5.dts):
root@letux:~# dmesg|fgrep iio [ 6.324584] input: iio-bridge: bmc150_accel as /devices/platform/68000000.ocp/48072000.i2c/i2c-1/1-0010/iio:device1/input/input5 [ 6.516632] input: iio-bridge: bno055 as /devices/platform/68000000.ocp/48072000.i2c/i2c-1/1-0029/iio:device3/input/input7 root@letux:~# evtest /dev/input/event5 | head -19 Input driver version is 1.0.1 Input device ID: bus 0x0 vendor 0x0 product 0x0 version 0x0 Input device name: "iio-bridge: bmc150_accel" Supported events: Event type 0 (EV_SYN) Event type 3 (EV_ABS) Event code 0 (ABS_X) Value 8 Min -511 Max 511 Event code 1 (ABS_Y) Value -44 Min -511 Max 511 Event code 2 (ABS_Z) Value -265 Min -511 Max 511 Properties: root@letux:~# evtest /dev/input/event7 | head -19 Input driver version is 1.0.1 Input device ID: bus 0x0 vendor 0x0 product 0x0 version 0x0 Input device name: "iio-bridge: bno055" Supported events: Event type 0 (EV_SYN) Event type 3 (EV_ABS) Event code 0 (ABS_X) Value -6 Min -511 Max 511 Event code 1 (ABS_Y) Value 17 Min -511 Max 511 Event code 2 (ABS_Z) Value -250 Min -511 Max 511 Properties: root@letux:~#
Although the sensor chips are mounted with different axis orientation, the application of the mount matrix provides equivalent (despite noise and precision) information on device orientation.
Signed-off-by: H. Nikolaus Schaller hns@goldelico.com
--- V1: initial RFC version V2: - rework based on comments by Jonathan Cameron - mainly: use input_polldev instead of using own polling timer - no need for checking number of open()/close() - no need for locks (already handled by input framework) V3: - use new iio_dev->input_mapping instead of mis-using iio_dev->private - removed some spurious printk from debugging - collect channels first and then register them all in one step - fix issue with unsigned int type propagation in atofix() - simplify code for handling negative numbers - fix sequence in unregister --- drivers/iio/Kconfig | 8 + drivers/iio/Makefile | 1 + drivers/iio/industrialio-core.c | 12 ++ drivers/iio/industrialio-inputbridge.c | 270 +++++++++++++++++++++++++ drivers/iio/industrialio-inputbridge.h | 28 +++ include/linux/iio/iio.h | 4 + 6 files changed, 323 insertions(+) create mode 100644 drivers/iio/industrialio-inputbridge.c create mode 100644 drivers/iio/industrialio-inputbridge.h
diff --git a/drivers/iio/Kconfig b/drivers/iio/Kconfig index d08aeb41cd07..2f0295da6ebc 100644 --- a/drivers/iio/Kconfig +++ b/drivers/iio/Kconfig @@ -68,6 +68,14 @@ config IIO_TRIGGERED_EVENT help Provides helper functions for setting up triggered events.
+config IIO_INPUT_BRIDGE + depends on INPUT + bool "Enable accelerometer bridge to input driver" + help + Provides a /dev/input/event* device for accelerometers + to use as a 3D input device, e.g. for gaming or auto-rotation + of screen contents. + source "drivers/iio/accel/Kconfig" source "drivers/iio/adc/Kconfig" source "drivers/iio/afe/Kconfig" diff --git a/drivers/iio/Makefile b/drivers/iio/Makefile index cb5993251381..d695e5a27da5 100644 --- a/drivers/iio/Makefile +++ b/drivers/iio/Makefile @@ -7,6 +7,7 @@ obj-$(CONFIG_IIO) += industrialio.o industrialio-y := industrialio-core.o industrialio-event.o inkern.o industrialio-$(CONFIG_IIO_BUFFER) += industrialio-buffer.o industrialio-$(CONFIG_IIO_TRIGGER) += industrialio-trigger.o +industrialio-$(CONFIG_IIO_INPUT_BRIDGE) += industrialio-inputbridge.o
obj-$(CONFIG_IIO_CONFIGFS) += industrialio-configfs.o obj-$(CONFIG_IIO_SW_DEVICE) += industrialio-sw-device.o diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c index 4700fd5d8c90..81f412b41a78 100644 --- a/drivers/iio/industrialio-core.c +++ b/drivers/iio/industrialio-core.c @@ -29,6 +29,7 @@ #include <linux/iio/iio.h> #include "iio_core.h" #include "iio_core_trigger.h" +#include "industrialio-inputbridge.h" #include <linux/iio/sysfs.h> #include <linux/iio/events.h> #include <linux/iio/buffer.h> @@ -1723,6 +1724,15 @@ int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod) if (ret < 0) goto error_unreg_eventset;
+ ret = iio_device_register_inputbridge(indio_dev); + if (ret) { + dev_err(indio_dev->dev.parent, + "Failed to register as input driver\n"); + device_del(&indio_dev->dev); + + return ret; + } + return 0;
error_unreg_eventset: @@ -1745,6 +1755,8 @@ void iio_device_unregister(struct iio_dev *indio_dev) { mutex_lock(&indio_dev->info_exist_lock);
+ iio_device_unregister_inputbridge(indio_dev); + cdev_device_del(&indio_dev->chrdev, &indio_dev->dev);
iio_device_unregister_debugfs(indio_dev); diff --git a/drivers/iio/industrialio-inputbridge.c b/drivers/iio/industrialio-inputbridge.c new file mode 100644 index 000000000000..592d5ee91a30 --- /dev/null +++ b/drivers/iio/industrialio-inputbridge.c @@ -0,0 +1,270 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * The Industrial I/O core, bridge to input devices + * + * Copyright (c) 2016-2019 Golden Delicious Computers GmbH&Co. KG + */ + +#include <linux/iio/consumer.h> +#include <linux/iio/iio.h> +#include <linux/iio/types.h> +#include <linux/input.h> +#include <linux/input-polldev.h> +#include <linux/module.h> +#include <linux/slab.h> + +#include "industrialio-inputbridge.h" + +/* currently, only polling is implemented */ +#define POLLING_MSEC 100 + +struct iio_input_map { + struct input_polled_dev *poll_dev; /* the input device */ + struct iio_channel channels[3]; /* x, y, z channels */ + struct matrix { + int mxx, myx, mzx; /* fixed point mount-matrix */ + int mxy, myy, mzy; + int mxz, myz, mzz; + } matrix; +}; + +static inline struct iio_input_map *to_iio_input_map( + struct iio_channel *channel) +{ + return (struct iio_input_map *) channel->data; +} + +/* minimum and maximum range we want to report */ +#define ABSMAX_ACC_VAL (512 - 1) +#define ABSMIN_ACC_VAL -(ABSMAX_ACC_VAL) + +/* scale processed iio values so that 1g maps to ABSMAX_ACC_VAL / 2 */ +#define SCALE ((100 * ABSMAX_ACC_VAL) / (2 * 981)) + +/* + * convert float string to scaled fixed point format, e.g. + * 1 -> 1000 (value passed as unit) + * 1.23 -> 1230 + * 0.1234 -> 123 + * -.01234 -> -12 + */ + +static int32_t atofix(const char *str, uint32_t unit) +{ + int32_t mantissa = 0; + bool decimal = false; + int divisor = 1; + + if (*str == '-') + divisor = -1, str++; + while (*str && abs(divisor) < unit) { + if (*str >= '0' && *str <= '9') { + mantissa = 10 * mantissa + (*str - '0'); + if (decimal) + divisor *= 10; + } else if (*str == '.') + decimal = true; + else + return 0; /* error */ + str++; + } + + return (mantissa * (int32_t) unit) / divisor; +} + +static void iio_apply_matrix(struct matrix *m, int *in, int *out, int unit) +{ + /* apply mount matrix */ + out[0] = (m->mxx * in[0] + m->myx * in[1] + m->mzx * in[2]) / unit; + out[1] = (m->mxy * in[0] + m->myy * in[1] + m->mzy * in[2]) / unit; + out[2] = (m->mxz * in[0] + m->myz * in[1] + m->mzz * in[2]) / unit; +} + +#define FIXED_POINT_UNIT 1000 /* seems reasonable for accelerometer input */ + +static void iio_accel_poll(struct input_polled_dev *dev) +{ + struct iio_input_map *map = dev->private; + struct input_dev *input = dev->input; + + int values[3]; /* values while processing */ + int aligned_values[3]; /* mount matrix applied */ + + int cindex = 0; + + while (cindex < ARRAY_SIZE(values)) { + struct iio_channel *channel = + &map->channels[cindex]; + int val; + int ret; + + if (!channel) { + values[cindex] = 0; + continue; + } + + ret = iio_read_channel_raw(channel, &val); + + if (ret < 0) { + pr_err("%s(): channel read error %d\n", + __func__, cindex); + return; + } + + ret = iio_convert_raw_to_processed(channel, val, + &values[cindex], SCALE); + + if (ret < 0) { + pr_err("%s(): channel processing error\n", + __func__); + return; + } + + cindex++; + } + + iio_apply_matrix(&map->matrix, values, aligned_values, FIXED_POINT_UNIT); + + input_report_abs(input, ABS_X, aligned_values[0]); + input_report_abs(input, ABS_Y, aligned_values[1]); + input_report_abs(input, ABS_Z, aligned_values[2]); + input_sync(input); +} + +static int iio_input_register_accel_channels(struct iio_dev *indio_dev, + const struct iio_chan_spec *chan[], int num_channels) +{ /* we found some accelerometer channel */ + int ret; + int cindex; + struct input_polled_dev *poll_dev; + struct iio_input_map *map = indio_dev->input_mapping; + const struct iio_chan_spec_ext_info *ext_info; + + if (unlikely(map)) + return -EINVAL; /* already registered */ + + if (num_channels < 1) + return 0; /* silently ignore */ + + map = devm_kzalloc(&indio_dev->dev, sizeof(struct iio_input_map), GFP_KERNEL); + if (!map) + return -ENOMEM; + + indio_dev->input_mapping = map; + + poll_dev = devm_input_allocate_polled_device(&indio_dev->dev); + if (!poll_dev) + return -ENOMEM; + + poll_dev->private = map; + poll_dev->poll = iio_accel_poll; + poll_dev->poll_interval = POLLING_MSEC; + + poll_dev->input->name = kasprintf(GFP_KERNEL, "iio-bridge: %s", + indio_dev->name); + poll_dev->input->phys = kasprintf(GFP_KERNEL, "iio:device%d", + indio_dev->id); + +// do we need something like this? +// poll_dev->input->id.bustype = BUS_IIO; +// poll_dev->input->id.vendor = 0x0001; +// poll_dev->input->id.product = 0x0001; +// poll_dev->input->id.version = 0x0001; + + set_bit(INPUT_PROP_ACCELEROMETER, poll_dev->input->propbit); + poll_dev->input->evbit[0] = BIT_MASK(EV_ABS); + input_alloc_absinfo(poll_dev->input); + input_set_abs_params(poll_dev->input, ABS_X, ABSMIN_ACC_VAL, + ABSMAX_ACC_VAL, 0, 0); + input_set_abs_params(poll_dev->input, ABS_Y, ABSMIN_ACC_VAL, + ABSMAX_ACC_VAL, 0, 0); + input_set_abs_params(poll_dev->input, ABS_Z, ABSMIN_ACC_VAL, + ABSMAX_ACC_VAL, 0, 0); + + map->poll_dev = poll_dev; + + ret = input_register_polled_device(poll_dev); + + if (ret < 0) { + kfree(poll_dev->input->name); + kfree(poll_dev->input->phys); + return ret; + } + + /* assume all channels of a device share the same matrix */ + + ext_info = chan[0]->ext_info; + while (ext_info && ext_info->name) { + if (strcmp(ext_info->name, "mount_matrix") == 0) + break; /* found */ + ext_info++; + } + + if (ext_info && ext_info->name) { + uintptr_t priv = ext_info->private; + const struct iio_mount_matrix *mtx; + + mtx = ((iio_get_mount_matrix_t *) priv)(indio_dev, + chan[0]); + + map->matrix.mxx = atofix(mtx->rotation[0], FIXED_POINT_UNIT); + map->matrix.myx = atofix(mtx->rotation[1], FIXED_POINT_UNIT); + map->matrix.mzx = atofix(mtx->rotation[2], FIXED_POINT_UNIT); + map->matrix.mxy = atofix(mtx->rotation[3], FIXED_POINT_UNIT); + map->matrix.myy = atofix(mtx->rotation[4], FIXED_POINT_UNIT); + map->matrix.mzy = atofix(mtx->rotation[5], FIXED_POINT_UNIT); + map->matrix.mxz = atofix(mtx->rotation[6], FIXED_POINT_UNIT); + map->matrix.myz = atofix(mtx->rotation[7], FIXED_POINT_UNIT); + map->matrix.mzz = atofix(mtx->rotation[8], FIXED_POINT_UNIT); + } else { + map->matrix.mxx = FIXED_POINT_UNIT; + map->matrix.myx = 0; + map->matrix.mzx = 0; + map->matrix.mxy = 0; + map->matrix.myy = FIXED_POINT_UNIT; + map->matrix.mzy = 0; + map->matrix.mxz = 0; + map->matrix.myz = 0; + map->matrix.mzz = FIXED_POINT_UNIT; + } + + for (cindex = 0; cindex < ARRAY_SIZE(map->channels); cindex++) { + if (cindex < num_channels) + map->channels[cindex].channel = chan[cindex]; + map->channels[cindex].indio_dev = indio_dev; + map->channels[cindex].data = map; + } + + return 0; +} + +int iio_device_register_inputbridge(struct iio_dev *indio_dev) +{ + int cindex; + int num_channels = 0; + const struct iio_chan_spec *channels[3]; + + for (cindex = 0; cindex < indio_dev->num_channels; cindex++) { + const struct iio_chan_spec *chan = + &indio_dev->channels[cindex]; + + if (chan->type == IIO_ACCEL && num_channels < ARRAY_SIZE(channels)) + channels[num_channels++] = chan; + } + + return iio_input_register_accel_channels(indio_dev, channels, num_channels); +} + +void iio_device_unregister_inputbridge(struct iio_dev *indio_dev) +{ + struct iio_input_map *map = iio_device_get_drvdata(indio_dev); + struct input_dev *input = map->poll_dev->input; + + input_unregister_polled_device(map->poll_dev); + kfree(input->name); + kfree(input->phys); +} + +MODULE_AUTHOR("H. Nikolaus Schaller hns@goldelico.com"); +MODULE_DESCRIPTION("Bridge to present Industrial I/O accelerometers as properly oriented Input devices"); +MODULE_LICENSE("GPL v2"); diff --git a/drivers/iio/industrialio-inputbridge.h b/drivers/iio/industrialio-inputbridge.h new file mode 100644 index 000000000000..1363b10ab3f7 --- /dev/null +++ b/drivers/iio/industrialio-inputbridge.h @@ -0,0 +1,28 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * The Industrial I/O core, bridge to input devices + * + * Copyright (c) 2016-2019 Golden Delicious Computers GmbH&Co. KG + * + * This program is free software; you can redistribute it and/or modify it + * under the terms of the GNU General Public License version 2 as published by + * the Free Software Foundation. + */ + +#if defined(CONFIG_IIO_INPUT_BRIDGE) + +extern int iio_device_register_inputbridge(struct iio_dev *indio_dev); +extern void iio_device_unregister_inputbridge(struct iio_dev *indio_dev); + +#else + +static inline int iio_device_register_inputbridge(struct iio_dev *indio_dev) +{ + return 0; +} + +static inline void iio_device_unregister_inputbridge(struct iio_dev *indio_dev) +{ +} + +#endif diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h index a74cb177dc6f..a4d2f11384e9 100644 --- a/include/linux/iio/iio.h +++ b/include/linux/iio/iio.h @@ -524,6 +524,7 @@ struct iio_buffer_setup_ops { * @flags: [INTERN] file ops related flags including busy flag. * @debugfs_dentry: [INTERN] device specific debugfs dentry. * @cached_reg_addr: [INTERN] cached register address for debugfs reads. + * @input_mapping: [INTERN] mapping for input device */ struct iio_dev { int id; @@ -570,6 +571,9 @@ struct iio_dev { struct dentry *debugfs_dentry; unsigned cached_reg_addr; #endif +#if defined(CONFIG_IIO_INPUT_BRIDGE) + void *input_mapping; +#endif };
const struct iio_chan_spec
Having written a "bridge" myself (I called it a "proxy"[1]), I have a few comments.
[1]: https://github.com/hadess/iio-sensor-proxy
Let's start with the easy ones ;) there's a typo in the subject line.
The subject line also says "optionally" but there doesn't seem to be any ways to disable the feature if it's shipped by the kernel used.
On Mon, 2019-04-15 at 23:05 +0200, H. Nikolaus Schaller wrote:
Some user spaces (e.g. some Android devices) use /dev/input/event* for handling the 3D position of the device with respect to the center of gravity (earth). This can be used for gaming input, auto-rotation of screens etc.
This interface should be the standard for such use cases because it is an abstraction of how orientation data is acquired from sensor chips. Sensor chips may be connected through different interfaces and in different positions. They may also have different parameters. And, if a chip is replaced by a different one, the values reported by the device position interface should remain the same, provided the device tree reflects the changed chip.
I don't understand this section of the commit message. The IIO drivers are already that abstraction interface, no?
This did initially lead to input accelerometer drivers like drivers/input/misc/bma150.c or drivers/misc/lis3lv02d/
But nowadays, new accelerometer chips mostly get iio drivers and rarely input drivers.
Therefore we need something like a protocol stack which bridges raw data and input devices. It can be seen as a similar layering like TCP/IP vs. bare Ethernet. Or keyboard input events vs. raw gpio or raw USB access.
This can be done in user-space, reading the data from the IIO driver, and using uinput to feed it back. Why is doing this at the kernel level better?
This patch bridges the gap between raw iio data and the input device abstraction so that accelerometer measurements can additionally be presented as X/Y/Z accelerometer channels (INPUT_PROP_ACCELEROMETER) through /dev/input/event*.
There are no special requirements or changes needed for an iio driver.
The user-space daemon I wrote supports both IIO drivers and input drivers for accelerometers. How do I know from user-space whether a device is proxied or not?
There is no need to define a mapping (e.g. in device tree).
This driver simply collects the first 3 accelerometer channels as X, Y and Z. If only 1 or 2 channels are available, they are used for X and Y only. Additional channels are ignored.
In what cases are 2 dimensional accelerometers used?
Scaling is done automatically so that 1g is represented by value 256 and range is assumed to be -511 .. +511 which gives a reasonable precision as an input device.
If a mount-matrix is provided by the iio driver, it is also taken into account so that the input event automatically gets the correct orientation with respect to the device.
If this extension is not configured into the kernel it takes no resources (except source code).
If it is configured, but there is no accelerometer, there is only a tiny penalty for scanning for accelerometer channels once during probe of each iio device.
If it runs, the driver polls the device(s) once every 100 ms. A mode where the iio device defines the update rate is not implemented and for further study.
If there is no user-space client, polling is not running.
Is the bridge going to modify the IIO device's settings behind other possible consumer's backs, such as threshold values, and triggers?
The driver is capable to handle multiple iio accelerometers and they are presented by unique /dev/input/event* files. The iio chip name is used to define the input device name so that it can be identified (e.g. by udev rules or evtest).
As you can probably guess, I'm not overly enthusiastic about this piece of code. If it had existed 5 years ago, I probably wouldn't have written iio-sensor-proxy, but then somebody else would have had to for the rest of the IIO sensors that can be consumed.
To me, this bridge has all the drawbacks of a simple user-space implementation using uinput, without much of the benefits of being an exclusive user of the IIO accelerometers, such as being able to change the update rate, or using triggers depending on the usage.
What am I missing? Why shouldn't this live in user-space?
Cheers
Hi Bastien,
Am 16.04.2019 um 18:04 schrieb Bastien Nocera hadess@hadess.net:
Having written a "bridge" myself (I called it a "proxy"[1]), I have a few comments.
Nice work!
Let's start with the easy ones ;) there's a typo in the subject line.
The subject line also says "optionally" but there doesn't seem to be any ways to disable the feature if it's shipped by the kernel used.
Well, the "optionally" refers to that this can be completely disabled by a Kconfig option. Maybe it is the wrong wording for this and should be changed.
On Mon, 2019-04-15 at 23:05 +0200, H. Nikolaus Schaller wrote:
Some user spaces (e.g. some Android devices) use /dev/input/event* for handling the 3D position of the device with respect to the center of gravity (earth). This can be used for gaming input, auto-rotation of screens etc.
This interface should be the standard for such use cases because it is an abstraction of how orientation data is acquired from sensor chips. Sensor chips may be connected through different interfaces and in different positions. They may also have different parameters. And, if a chip is replaced by a different one, the values reported by the device position interface should remain the same, provided the device tree reflects the changed chip.
I don't understand this section of the commit message. The IIO drivers are already that abstraction interface, no?
IIO is also some abstraction but a different one than input accelerometers.
IIO reports physical measurement data in some standardized way reporting value, scale, units, type of measurement. But this has no inherent purpose.
Accelerator input events are something different. They report the orientation of a handheld device in space relative to center of earth. They may be implemented through iio drivers but do not need to.
You will find several non-iio accelerometer drivers in drivers/input/misc and elsewhere.
This did initially lead to input accelerometer drivers like drivers/input/misc/bma150.c or drivers/misc/lis3lv02d/
But nowadays, new accelerometer chips mostly get iio drivers and rarely input drivers.
Therefore we need something like a protocol stack which bridges raw data and input devices. It can be seen as a similar layering like TCP/IP vs. bare Ethernet. Or keyboard input events vs. raw gpio or raw USB access.
This can be done in user-space, reading the data from the IIO driver, and using uinput to feed it back. Why is doing this at the kernel level better?
Well, I'd estimate that >80% of the current kernel could be done in user-space (but not at the same speed/quality).
E.g. TCP could most likely be done by directly accessing the Ethernet layer and providing other processes access through named pipes instead of sockets.
But usually a user-space daemon feeding things back into the kernel is slower (because it is scheduled differently) and needs more resources for running the process and IPC and is less protected against hickups and deadlocks.
Two more aspects come to my mind from reading your project page:
a) "It requires libgudev and systemd" b) "Note that a number of kernel bugs will prevent it from working correctly"
a) this makes quite significant assumptions about the user-space while a kernel driver can be kept independent of this b) if it is in-kernel it will be kept in sync with kernel changes and such bugs are less likely
This patch bridges the gap between raw iio data and the input device abstraction so that accelerometer measurements can additionally be presented as X/Y/Z accelerometer channels (INPUT_PROP_ACCELEROMETER) through /dev/input/event*.
There are no special requirements or changes needed for an iio driver.
The user-space daemon I wrote supports both IIO drivers and input drivers for accelerometers. How do I know from user-space whether a device is proxied or not?
Since my proposal does not stop direct iio access, I assume that your daemon will simply continue to work as is.
There is no need to define a mapping (e.g. in device tree).
This driver simply collects the first 3 accelerometer channels as X, Y and Z. If only 1 or 2 channels are available, they are used for X and Y only. Additional channels are ignored.
In what cases are 2 dimensional accelerometers used?
I don't know. This is just a description that there will be a graceful behavior, if some accelerometer has less or more than 3 channels. So the driver will segfault or panic the kernel...
Scaling is done automatically so that 1g is represented by value 256 and range is assumed to be -511 .. +511 which gives a reasonable precision as an input device.
If a mount-matrix is provided by the iio driver, it is also taken into account so that the input event automatically gets the correct orientation with respect to the device.
If this extension is not configured into the kernel it takes no resources (except source code).
If it is configured, but there is no accelerometer, there is only a tiny penalty for scanning for accelerometer channels once during probe of each iio device.
If it runs, the driver polls the device(s) once every 100 ms. A mode where the iio device defines the update rate is not implemented and for further study.
If there is no user-space client, polling is not running.
Is the bridge going to modify the IIO device's settings behind other possible consumer's backs, such as threshold values, and triggers?
No. The bridge only transforms values.
For other parameters it takes what the iio device has been initialized to Or if they are changed dynamically through the iio API, it uses new parameters.
The driver is capable to handle multiple iio accelerometers and they are presented by unique /dev/input/event* files. The iio chip name is used to define the input device name so that it can be identified (e.g. by udev rules or evtest).
As you can probably guess, I'm not overly enthusiastic about this piece of code. If it had existed 5 years ago, I probably wouldn't have written iio-sensor-proxy, but then somebody else would have had to for the rest of the IIO sensors that can be consumed.
I have looked into your work and get the impression that my proposal is not contradicting your work.
You still need mechanisms to rotate e.g. the display if the device is rotated. This is not done by this kernel code.
This is a similar situation that a keyboard driver reports key event codes but does not draw glyphs.
The bridge provides a different (and well established) higher-level API for accelerometers than iio.
So you may check if it simplifies your code by using this higher-layer interface (input event). From my rough check it appears to me that you can for example remove the mount-matrix handling from your code because it would be done by this bridge.
To me, this bridge has all the drawbacks of a simple user-space implementation using uinput, without much of the benefits of being an exclusive user of the IIO accelerometers, such as being able to change the update rate, or using triggers depending on the usage.
What am I missing? Why shouldn't this live in user-space?
That's a pretty biased question...
It's a matter of philosophy whether you want a microkernel + user-space daemons or a kernel that prebakes many things to make user-space daemons easier or even superfluous.
In the other extreme you could even get rid of iio and directly access the sensors through /dev/i2c from user-space daemon.
So I can't give you a technical answer for that and since I am not a maintainer I can't answer it at all.
BR and thanks, Nikolaus
On Tue, 2019-04-16 at 21:33 +0200, H. Nikolaus Schaller wrote:
Hi Bastien,
Am 16.04.2019 um 18:04 schrieb Bastien Nocera hadess@hadess.net: This can be done in user-space, reading the data from the IIO driver, and using uinput to feed it back. Why is doing this at the kernel level better?
Well, I'd estimate that >80% of the current kernel could be done in user-space (but not at the same speed/quality).
E.g. TCP could most likely be done by directly accessing the Ethernet layer and providing other processes access through named pipes instead of sockets.
But usually a user-space daemon feeding things back into the kernel is slower (because it is scheduled differently) and needs more resources for running the process and IPC and is less protected against hickups and deadlocks.
This is mostly irrelevant for the amount of data we're treating, but it doesn't matter too much.
Two more aspects come to my mind from reading your project page:
a) "It requires libgudev and systemd" b) "Note that a number of kernel bugs will prevent it from working correctly"
a) this makes quite significant assumptions about the user-space while a kernel driver can be kept independent of this
It's made for modern desktop OSes/"traditional" Linux. I don't think that those 2 libraries are problematic dependencies unless you're on Android, where a replacement could be implemented or iio-sensor-proxy modified for that use case.
b) if it is in-kernel it will be kept in sync with kernel changes and such bugs are less likely
No they're not. This warning was because 1) drivers sometimes have bugs 2) user-space sometimes has bugs 3) user-space sometimes causes the kernel to have bugs.
The 2 significant breakages for iio-sensor-proxy were caused by runtime PM bugs in the hid-sensor-hub driver, and in the USB core. I doubt a kernel-space implementation would have been able to magically fix those bugs unfortunately.
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Bastien Nocera -
H. Nikolaus Schaller