Hi,
[ I added Daniel, Marc & Steven to cc: ]
On Wednesday, May 10, 2017 10:24:14 AM Linus Walleij 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/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
>From looking at the code it seems that bounce buffering should
be always beneficial to MMC performance when host->max_segs == 1.
It would be useful to know why these specific defconfigs
disable bounce buffering (to save memory?). Maybe the defaults
should be changed nowadays?
[...]
> drivers/mmc/core/Kconfig | 18 ------------------
> drivers/mmc/core/queue.c | 15 +--------------
> drivers/mmc/host/cavium.c | 3 +++
> drivers/mmc/host/pxamci.c | 6 ++++++
> include/linux/mmc/host.h | 1 +
> 5 files changed, 11 insertions(+), 32 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..545466342fb1 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->disable_bounce)
> 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..66066f73e477 100644
> --- a/drivers/mmc/host/cavium.c
> +++ b/drivers/mmc/host/cavium.c
> @@ -1050,6 +1050,9 @@ int cvm_mmc_of_slot_probe(struct device *dev, struct
> cvm_mmc_host *host)
> else
> mmc->max_segs = 1;
>
> + /* Disable bounce buffers for max_segs = 1 */
> + mmc->disable_bounce = true;
> +
> /* DMA size field can address up to 8 MB */
> mmc->max_seg_size = 8 * 1024 * 1024;
> mmc->max_req_size = mmc->max_seg_size;
> diff --git a/drivers/mmc/host/pxamci.c b/drivers/mmc/host/pxamci.c
> index c763b404510f..d3b5e6376504 100644
> --- a/drivers/mmc/host/pxamci.c
> +++ b/drivers/mmc/host/pxamci.c
> @@ -666,6 +666,12 @@ static int pxamci_probe(struct platform_device *pdev)
> mmc->max_segs = NR_SG;
>
> /*
> + * This architecture used to disable bounce buffers through its
> + * defconfig, now it is done at runtime as a host property.
> + */
> + mmc->disable_bounce = true;
> +
> + /*
> * Our hardware DMA can handle a maximum of one page per SG entry.
> */
> mmc->max_seg_size = PAGE_SIZE;
> 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 */
> 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 */
Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics
