On 3 July 2018 at 20:45, Belisko Marek marek.belisko@gmail.com wrote:
Hi Ulf,
CC'ed pyra kernel guys
On Tue, Jul 3, 2018 at 11:23 AM Ulf Hansson ulf.hansson@linaro.org wrote:
On 3 July 2018 at 08:47, Belisko Marek marek@goldelico.com wrote:
Hello,
on pyra device [0] we're using txs02612 sdio expander to have possibility to connect SD card and also eMMC. We have some version of driver (gpio for switching bus is controlled via sysfs) which probably won't accepted. We want to do it correct way so we was thinking about same approach as i2c-mux is using (i2c device connected on mux buses are transparent to user so switching is done completely in driver). I would like to maybe get some initial thoughts if this is good idea to go this way and don't be a problem for future upstreaming. Thanks for any comments.
Sorry, but I doubt we ever get to add proper upstream support for mmc hosts managing multiple slots. Simply because all the efforts it takes isn't worth it.
Today there are some host trying to support this, but those are hacks, which is working very poorly. I don't want to maintain more of those, then it's better those are managed by vendors out of tree.
Thanks for quick reply.
(just copy part of internal discussion): My approach would be that the driver registers two new host controllers, e.g. like mmc1 and mmc2 on the omap5.
I.e. calls in its probe function
mmc[i] = mmc_alloc_host(sizeof(struct omap_hsmmc_host), &pdev->dev); mmc[i]->ops = txs02612_ops; // other setup mmc_add_host(mmc[i]);twice for each of the two txs channels. This should create two new user-space mmc ports that receive requests and other callbacks.
It then should takes control of the real host controller to sit in the middle.
So basically we should do something like:
host_mmc = sarch_by_of_handle(...)And then make the txs02612_ops like:
txs02612_request(mmc, ...) { find out which port if(different from current) { try to get mutex throw the switch } host_mmc->request(host_mmc, ...) if(was different) release the mutex }in this case we don't need to touch mmc core and also bus locking will be done in driver. Do you think we can get some success with upstreaming by this approach?
No.
Thanks.
There have been several discussions on this topic, this is just one that I found while searching. https://lkml.org/lkml/2017/10/6/710
Did you read this?
If you need any further explainations, please just ask.
Let me elaborate a bit on the problems.
1. Serialization of requests (several request sent after each other, with delays, changing clocks etc), is managed by the core, because it implements the SD/SDIO/(e)MMC specs. This does not belong in a host driver. -> A kind of mmc bus slot lock is needed in the core. Not a big deal, we can probably hide it below mmc_claim|release_host().
2. What if there is an SD card that can cope with high-speed mode in one slot with a clock freq at 50 MHz. Then in the other slot, there is an eMMC using HS200 mode, running at 200 MHz. Can the HW guarantee all kind of different combinations, with clock glitches etc?
3. The biggest problem: You can *not* provide any service guarantees in regards to latency or bandwidth for any of the storage media in the slots, because of the bus slot lock. That because the upper layers runs I/O scheduling separately for each storage device, thus request for one slot can starve requests on another.
Kind regards Uffe