• Baolin Wang's avatar
    mmc: Add MMC host software queue support · 511ce378
    Baolin Wang authored
    Now the MMC read/write stack will always wait for previous request is
    completed by mmc_blk_rw_wait(), before sending a new request to hardware,
    or queue a work to complete request, that will bring context switching
    overhead and spend some extra time to poll the card for busy completion
    for I/O writes via sending CMD13, especially for high I/O per second
    rates, to affect the IO performance.
    
    Thus this patch introduces MMC software queue interface based on the
    hardware command queue engine's interfaces, which is similar with the
    hardware command queue engine's idea, that can remove the context
    switching. Moreover we set the default queue depth as 64 for software
    queue, which allows more requests to be prepared, merged and inserted
    into IO scheduler to improve performance, but we only allow 2 requests
    in flight, that is enough to let the irq handler always trigger the
    next request without a context switch, as well as avoiding a long latency.
    
    Moreover the host controller should support HW busy detection for I/O
    operations when enabling the host software queue. That means, the host
    controller must not complete a data transfer request, until after the
    card stops signals busy.
    
    From the fio testing data in cover letter, we can see the software
    queue can improve some performance with 4K block size, increasing
    about 16% for random read, increasing about 90% for random write,
    though no obvious improvement for sequential read and write.
    
    Moreover we can expand the software queue interface to support MMC
    packed request or packed command in future.
    Reviewed-by: default avatarArnd Bergmann <arnd@arndb.de>
    Signed-off-by: default avatarBaolin Wang <baolin.wang@linaro.org>
    Signed-off-by: default avatarBaolin Wang <baolin.wang7@gmail.com>
    Link: https://lore.kernel.org/r/4409c1586a9b3ed20d57ad2faf6c262fc3ccb6e2.1581478568.git.baolin.wang7@gmail.comSigned-off-by: default avatarUlf Hansson <ulf.hansson@linaro.org>
    511ce378
block.c 76.4 KB