Re: [PATCH 1/5] mmc: core: Delete bounce buffer Kconfig option

On Mon, May 15, 2017 at 4:04 PM, Bartlomiej Zolnierkiewicz
[off-list ref] wrote:

[ I added Daniel, Marc & Steven to cc: ]

quoted

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

[...]

quoted

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?

I agree. But I can't test on pxamci and cavium so I would
appreciate if the driver maintainers try to remove the
flag (MMC_CAP_NO_BOUNCE_BUFF in the v2 patch set)
and see what happens.

I would be happy to cut this special flag altogether, but I am
also afraid of screwing up some config :/

Yours,
Linus Walleij