• Linus Walleij's avatar
    mmc: Delete bounce buffer handling · de3ee99b
    Linus Walleij authored
    In may, Steven sent a patch deleting the bounce buffer handling
    and the CONFIG_MMC_BLOCK_BOUNCE option.
    
    I chose the less invasive path of making it a runtime config
    option, and we merged that successfully for kernel v4.12.
    
    The code is however just standing in the way and taking up
    space for seemingly no gain on any systems in wide use today.
    
    Pierre says the code was there to improve speed on TI SDHCI
    controllers on certain HP laptops and possibly some Ricoh
    controllers as well. Early SDHCI controllers lacked the
    scatter-gather feature, which made software bounce buffers
    a significant speed boost.
    
    We are clearly talking about the list of SDHCI PCI-based
    MMC/SD card readers found in the pci_ids[] list in
    drivers/mmc/host/sdhci-pci-core.c.
    
    The TI SDHCI derivative is not supported by the upstream
    kernel. This leaves the Ricoh.
    
    What we can however notice is that the x86 defconfigs in the
    kernel did not enable CONFIG_MMC_BLOCK_BOUNCE option, which
    means that any such laptop would have to have a custom
    configured kernel to actually take advantage of this
    bounce buffer speed-up. It simply seems like there was
    a speed optimization for the Ricoh controllers that noone
    was using. (I have not checked the distro defconfigs but
    I am pretty sure the situation is the same there.)
    
    Bounce buffers increased performance on the OMAP HSMMC
    at one point, and was part of the original submission in
    commit a45c6cb8 ("[ARM] 5369/1: omap mmc: Add new
       omap hsmmc controller for 2430 and 34xx, v3")
    
    This optimization was removed in
    commit 0ccd76d4 ("omap_hsmmc: Implement scatter-gather
       emulation")
    which found that scatter-gather emulation provided even
    better performance.
    
    The same was introduced for SDHCI in
    commit 2134a922 ("sdhci: scatter-gather (ADMA) support")
    
    I am pretty positively convinced that software
    scatter-gather emulation will do for any host controller what
    the bounce buffers were doing. Essentially, the bounce buffer
    was a reimplementation of software scatter-gather-emulation in
    the MMC subsystem, and it should be done away with.
    
    Cc: Pierre Ossman <pierre@ossman.eu>
    Cc: Juha Yrjola <juha.yrjola@solidboot.com>
    Cc: Steven J. Hill <Steven.Hill@cavium.com>
    Cc: Shawn Lin <shawn.lin@rock-chips.com>
    Cc: Adrian Hunter <adrian.hunter@intel.com>
    Suggested-by: default avatarSteven J. Hill <Steven.Hill@cavium.com>
    Suggested-by: default avatarShawn Lin <shawn.lin@rock-chips.com>
    Signed-off-by: default avatarLinus Walleij <linus.walleij@linaro.org>
    Signed-off-by: default avatarUlf Hansson <ulf.hansson@linaro.org>
    de3ee99b
cavium.c 27.8 KB