On 30 March 2017 at 14:42, Arnd Bergmann <a...@arndb.de> wrote: > On Wed, Mar 29, 2017 at 5:09 AM, Linus Walleij <linus.wall...@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
