SDHCI changed from using a tasklet to finish requests, to using an IRQ
thread i.e. commit c07a48c2 ("mmc: sdhci: Remove finish_tasklet").
Because this increased the latency to complete requests, a preparatory
change was made to complete the request from the IRQ handler if
possible i.e. commit 19d2f695 ("mmc: sdhci: Call mmc_request_done()
from IRQ handler if possible").  That alleviated the situation for MMC
block devices because the MMC block driver makes use of mmc_pre_req()
and mmc_post_req() so that successful requests are completed in the IRQ
handler and any DMA unmapping is handled separately in mmc_post_req().
However SDIO was still affected, and an example has been reported with
up to 20% degradation in performance.

Looking at SDIO I/O helper functions, sdio_io_rw_ext_helper() appeared
to be a possible candidate for making use of asynchronous requests
within its I/O loops, but analysis revealed that these loops almost
never iterate more than once, so the complexity of the change would not
be warrented.

Instead, mmc_pre_req() and mmc_post_req() are added before and after I/O
submission (mmc_wait_for_req) in mmc_io_rw_extended().  This still has
the potential benefit of reducing the duration of interrupt handlers, as
well as addressing the latency issue for SDHCI.  It also seems a more
reasonable solution than forcing drivers to do everything in the IRQ
handler.
Reported-by: Dmitry Osipenko <digetx@gmail.com>
Fixes: c07a48c2 ("mmc: sdhci: Remove finish_tasklet")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200903082007.18715-1-adrian.hunter@intel.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>