On 10 May 2017 at 10:24, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> 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.
Thanks for the descriptive change-log!
[...]
> diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
> index 21385ac0c9b1..b53c0e18a33b 100644
> --- a/include/linux/mmc/host.h
> +++ b/include/linux/mmc/host.h
> @@ -310,6 +310,7 @@ struct mmc_host {
> /* host specific block data */
> unsigned int max_seg_size; /* see blk_queue_max_segment_size */
> unsigned short max_segs; /* see blk_queue_max_segments */
> + bool disable_bounce; /* disable bounce buffers */
Instead of adding a separate host configuration flag, let's use one of
capabilities bit-field instead (->caps or ->caps2). Then I suggest we
then add a new bit, MMC_CAP_NO_BOUNCE_BUFF.
> unsigned short unused;
> unsigned int max_req_size; /* maximum number of bytes in one req */
> unsigned int max_blk_size; /* maximum size of one mmc block */
> --
> 2.9.3
>
Kind regards
Uffe
