Re: [PATCH 1/6 v2] mmc: core: Delete bounce buffer Kconfig option

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On 18 May 2017 at 11:29, 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.
>
> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>

Thanks, the *series* applied for next! (Responding to patch1 as
couldn't find the cover-letter for v2).

Kind regards
Uffe

> ---
> ChangeLog v1->v2:
> - Instead of adding a new bool "disable_bounce" we use the host
>   caps variable, reuse the free bit 21 to indicate that bounce
>   buffers should be disabled on the host.
> ---
>  drivers/mmc/core/Kconfig  | 18 ------------------
>  drivers/mmc/core/queue.c  | 15 +--------------
>  drivers/mmc/host/cavium.c |  4 +++-
>  drivers/mmc/host/pxamci.c |  6 +++++-
>  include/linux/mmc/host.h  |  1 +
>  5 files changed, 10 insertions(+), 34 deletions(-)
>
> diff --git a/drivers/mmc/core/Kconfig b/drivers/mmc/core/Kconfig
> index fc1ecdaaa9ca..42e89060cd41 100644
> --- a/drivers/mmc/core/Kconfig
> +++ b/drivers/mmc/core/Kconfig
> @@ -61,24 +61,6 @@ config MMC_BLOCK_MINORS
>
>           If unsure, say 8 here.
>
> -config MMC_BLOCK_BOUNCE
> -       bool "Use bounce buffer for simple hosts"
> -       depends on MMC_BLOCK
> -       default y
> -       help
> -         SD/MMC is a high latency protocol where it is crucial to
> -         send large requests in order to get high performance. Many
> -         controllers, however, are restricted to continuous memory
> -         (i.e. they can't do scatter-gather), something the kernel
> -         rarely can provide.
> -
> -         Say Y here to help these restricted hosts by bouncing
> -         requests back and forth from a large buffer. You will get
> -         a big performance gain at the cost of up to 64 KiB of
> -         physical memory.
> -
> -         If unsure, say Y here.
> -
>  config SDIO_UART
>         tristate "SDIO UART/GPS class support"
>         depends on TTY
> diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c
> index 5c37b6be3e7b..70ba7f94c706 100644
> --- a/drivers/mmc/core/queue.c
> +++ b/drivers/mmc/core/queue.c
> @@ -219,7 +219,6 @@ static struct mmc_queue_req *mmc_queue_alloc_mqrqs(int qdepth)
>         return mqrq;
>  }
>
> -#ifdef CONFIG_MMC_BLOCK_BOUNCE
>  static int mmc_queue_alloc_bounce_bufs(struct mmc_queue_req *mqrq, int qdepth,
>                                        unsigned int bouncesz)
>  {
> @@ -258,7 +257,7 @@ static unsigned int mmc_queue_calc_bouncesz(struct mmc_host *host)
>  {
>         unsigned int bouncesz = MMC_QUEUE_BOUNCESZ;
>
> -       if (host->max_segs != 1)
> +       if (host->max_segs != 1 || (host->caps & MMC_CAP_NO_BOUNCE_BUFF))
>                 return 0;
>
>         if (bouncesz > host->max_req_size)
> @@ -273,18 +272,6 @@ static unsigned int mmc_queue_calc_bouncesz(struct mmc_host *host)
>
>         return bouncesz;
>  }
> -#else
> -static inline bool mmc_queue_alloc_bounce(struct mmc_queue_req *mqrq,
> -                                         int qdepth, unsigned int bouncesz)
> -{
> -       return false;
> -}
> -
> -static unsigned int mmc_queue_calc_bouncesz(struct mmc_host *host)
> -{
> -       return 0;
> -}
> -#endif
>
>  static int mmc_queue_alloc_sgs(struct mmc_queue_req *mqrq, int qdepth,
>                                int max_segs)
> diff --git a/drivers/mmc/host/cavium.c b/drivers/mmc/host/cavium.c
> index 58b51ba6aabd..9c1575f7c1fb 100644
> --- a/drivers/mmc/host/cavium.c
> +++ b/drivers/mmc/host/cavium.c
> @@ -1040,10 +1040,12 @@ int cvm_mmc_of_slot_probe(struct device *dev, struct cvm_mmc_host *host)
>          * We only have a 3.3v supply, we cannot support any
>          * of the UHS modes. We do support the high speed DDR
>          * modes up to 52MHz.
> +        *
> +        * Disable bounce buffers for max_segs = 1
>          */
>         mmc->caps |= MMC_CAP_MMC_HIGHSPEED | MMC_CAP_SD_HIGHSPEED |
>                      MMC_CAP_ERASE | MMC_CAP_CMD23 | MMC_CAP_POWER_OFF_CARD |
> -                    MMC_CAP_3_3V_DDR;
> +                    MMC_CAP_3_3V_DDR | MMC_CAP_NO_BOUNCE_BUFF;
>
>         if (host->use_sg)
>                 mmc->max_segs = 16;
> diff --git a/drivers/mmc/host/pxamci.c b/drivers/mmc/host/pxamci.c
> index c763b404510f..59ab194cb009 100644
> --- a/drivers/mmc/host/pxamci.c
> +++ b/drivers/mmc/host/pxamci.c
> @@ -702,7 +702,11 @@ static int pxamci_probe(struct platform_device *pdev)
>
>         pxamci_init_ocr(host);
>
> -       mmc->caps = 0;
> +       /*
> +        * This architecture used to disable bounce buffers through its
> +        * defconfig, now it is done at runtime as a host property.
> +        */
> +       mmc->caps = MMC_CAP_NO_BOUNCE_BUFF;
>         host->cmdat = 0;
>         if (!cpu_is_pxa25x()) {
>                 mmc->caps |= MMC_CAP_4_BIT_DATA | MMC_CAP_SDIO_IRQ;
> diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
> index 21385ac0c9b1..67f6abe5c3af 100644
> --- a/include/linux/mmc/host.h
> +++ b/include/linux/mmc/host.h
> @@ -270,6 +270,7 @@ struct mmc_host {
>  #define MMC_CAP_UHS_SDR50      (1 << 18)       /* Host supports UHS SDR50 mode */
>  #define MMC_CAP_UHS_SDR104     (1 << 19)       /* Host supports UHS SDR104 mode */
>  #define MMC_CAP_UHS_DDR50      (1 << 20)       /* Host supports UHS DDR50 mode */
> +#define MMC_CAP_NO_BOUNCE_BUFF (1 << 21)       /* Disable bounce buffers on host */
>  #define MMC_CAP_DRIVER_TYPE_A  (1 << 23)       /* Host supports Driver Type A */
>  #define MMC_CAP_DRIVER_TYPE_C  (1 << 24)       /* Host supports Driver Type C */
>  #define MMC_CAP_DRIVER_TYPE_D  (1 << 25)       /* Host supports Driver Type D */
> --
> 2.9.3
>
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