• Linus Walleij's avatar
    mmc: core: Delete bounce buffer Kconfig option · c3dccb74
    Linus Walleij authored
    This option is activated by all multiplatform configs and what
    not so we almost always have it turned on, and the memory it
    saves is negligible, even more so moving forward. The actual
    bounce buffer only gets allocated only when used, the only
    thing the ifdefs are saving is a little bit of code.
    
    It is highly improper to have this as a Kconfig option that
    get turned on by Kconfig, make this a pure runtime-thing and
    let the host decide whether we use bounce buffers. We add a
    new property "disable_bounce" to the host struct.
    
    Notice that mmc_queue_calc_bouncesz() already disables the
    bounce buffers if host->max_segs != 1, so any arch that has a
    maximum number of segments higher than 1 will have bounce
    buffers disabled.
    
    The option CONFIG_MMC_BLOCK_BOUNCE is default y so the
    majority of platforms in the kernel already have it on, and
    it then gets turned off at runtime since most of these have
    a host->max_segs > 1. The few exceptions that have
    host->max_segs == 1 and still turn off the bounce buffering
    are those that disable it in their defconfig.
    
    Those are the following:
    
    arch/arm/configs/colibri_pxa300_defconfig
    arch/arm/configs/zeus_defconfig
    - Uses MMC_PXA, drivers/mmc/host/pxamci.c
    - Sets host->max_segs = NR_SG, which is 1
    - This needs its bounce buffer deactivated so we set
      host->disable_bounce to true in the host driver
    
    arch/arm/configs/davinci_all_defconfig
    - Uses MMC_DAVINCI, drivers/mmc/host/davinci_mmc.c
    - This driver sets host->max_segs to MAX_NR_SG, which is 16
    - That means this driver anyways disabled bounce buffers
    - No special action needed for this platform
    
    arch/arm/configs/lpc32xx_defconfig
    arch/arm/configs/nhk8815_defconfig
    arch/arm/configs/u300_defconfig
    - Uses MMC_ARMMMCI, drivers/mmc/host/mmci.[c|h]
    - This driver by default sets host->max_segs to NR_SG,
      which is 128, unless a DMA engine is used, and in that case
      the number of segments are also > 1
    - That means this driver already disables bounce buffers
    - No special action needed for these platforms
    
    arch/arm/configs/sama5_defconfig
    - Uses MMC_SDHCI, MMC_SDHCI_PLTFM, MMC_SDHCI_OF_AT91, MMC_ATMELMCI
    - Uses drivers/mmc/host/sdhci.c
    - Normally sets host->max_segs to SDHCI_MAX_SEGS which is 128 and
      thus disables bounce buffers
    - Sets host->max_segs to 1 if SDHCI_USE_SDMA is set
    - SDHCI_USE_SDMA is only set by SDHCI on PCI adapers
    - That means that for this platform bounce buffers are already
      disabled at runtime
    - No special action needed for this platform
    
    arch/blackfin/configs/CM-BF533_defconfig
    arch/blackfin/configs/CM-BF537E_defconfig
    - Uses MMC_SPI (a simple MMC card connected on SPI pins)
    - Uses drivers/mmc/host/mmc_spi.c
    - Sets host->max_segs to MMC_SPI_BLOCKSATONCE which is 128
    - That means this platform already disables bounce buffers at
      runtime
    - No special action needed for these platforms
    
    arch/mips/configs/cavium_octeon_defconfig
    - Uses MMC_CAVIUM_OCTEON, drivers/mmc/host/cavium.c
    - Sets host->max_segs to 16 or 1
    - Setting host->disable_bounce to be sure for the 1 case
    
    arch/mips/configs/qi_lb60_defconfig
    - Uses MMC_JZ4740, drivers/mmc/host/jz4740_mmc.c
    - This sets host->max_segs to 128 so bounce buffers are
      already runtime disabled
    - No action needed for this platform
    
    It would be interesting to come up with a list of the platforms
    that actually end up using bounce buffers. I have not been
    able to infer such a list, but it occurs when
    host->max_segs == 1 and the bounce buffering is not explicitly
    disabled.
    Signed-off-by: default avatarLinus Walleij <linus.walleij@linaro.org>
    Signed-off-by: default avatarUlf Hansson <ulf.hansson@linaro.org>
    c3dccb74
pxamci.c 20.9 KB