Re: Outstanding MQ questions from MMC

[Ulf Hansson <ulf.hansson@linaro.org>]

On 30 March 2017 at 14:42, Arnd Bergmann <arnd@arndb.de> wrote:
> On Wed, Mar 29, 2017 at 5:09 AM, Linus Walleij <linus.walleij@linaro.org> wrote:
>> Hi folks,
>>
>> I earlier left some unanswered questions in my MMC to MQ conversion series
>> but I figured it is better if I collect them and ask the blk-mq
>> maintainers directly
>> how to deal with the following situations that occur in the MMC block layer:
>>
>>
>> 1. The current MMC code locks the host when the first request comes in
>> from blk_fetch_request() and unlocks it when blk_fetch_request() returns
>> NULL twice in a row. Then the polling thread terminated and is not restarted
>> until we get called by the mmc_request_fn.
>>
>> Host locking means that we will not send other commands to the MMC
>> card from i.e. userspace, which sometimes can send spurious stuff orthogonal
>> to the block layer. If the block layer has locked the host, userspace
>> has to wait
>> and vice versa. It is not a common contention point but it still happens.
>>
>> In MQ, I have simply locked the host on the first request and then I never
>> release it. Clearly this does not work. I am uncertain on how to handle this
>> and whether MQ has a way to tell us that the queue is empty so we may release
>> the host. I toyed with the idea to just set up a timer, but a "queue
>> empty" callback
>> from the block layer is what would be ideal.
>
> Would it be possible to change the userspace code to go through
> the block layer instead and queue a request there, to avoid having
> to lock the card at all?

That would be good from an I/O scheduling point of view, as it would
avoid one side being able to starve the other.

However, we would still need a lock, as we also have card detect work
queue, which also needs to claim the host when it polls for removable
cards.

Kind regards
Uffe