On Wed, Mar 29, 2017 at 5:09 AM, Linus Walleij <linus.walleij@xxxxxxxxxx> 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?
Arnd
