WSL2-Linux-Kernel/include/linux/mmc
Linus Walleij de3ee99b09 mmc: Delete bounce buffer handling
In may, Steven sent a patch deleting the bounce buffer handling
and the CONFIG_MMC_BLOCK_BOUNCE option.

I chose the less invasive path of making it a runtime config
option, and we merged that successfully for kernel v4.12.

The code is however just standing in the way and taking up
space for seemingly no gain on any systems in wide use today.

Pierre says the code was there to improve speed on TI SDHCI
controllers on certain HP laptops and possibly some Ricoh
controllers as well. Early SDHCI controllers lacked the
scatter-gather feature, which made software bounce buffers
a significant speed boost.

We are clearly talking about the list of SDHCI PCI-based
MMC/SD card readers found in the pci_ids[] list in
drivers/mmc/host/sdhci-pci-core.c.

The TI SDHCI derivative is not supported by the upstream
kernel. This leaves the Ricoh.

What we can however notice is that the x86 defconfigs in the
kernel did not enable CONFIG_MMC_BLOCK_BOUNCE option, which
means that any such laptop would have to have a custom
configured kernel to actually take advantage of this
bounce buffer speed-up. It simply seems like there was
a speed optimization for the Ricoh controllers that noone
was using. (I have not checked the distro defconfigs but
I am pretty sure the situation is the same there.)

Bounce buffers increased performance on the OMAP HSMMC
at one point, and was part of the original submission in
commit a45c6cb816 ("[ARM] 5369/1: omap mmc: Add new
   omap hsmmc controller for 2430 and 34xx, v3")

This optimization was removed in
commit 0ccd76d4c2 ("omap_hsmmc: Implement scatter-gather
   emulation")
which found that scatter-gather emulation provided even
better performance.

The same was introduced for SDHCI in
commit 2134a922c6 ("sdhci: scatter-gather (ADMA) support")

I am pretty positively convinced that software
scatter-gather emulation will do for any host controller what
the bounce buffers were doing. Essentially, the bounce buffer
was a reimplementation of software scatter-gather-emulation in
the MMC subsystem, and it should be done away with.

Cc: Pierre Ossman <pierre@ossman.eu>
Cc: Juha Yrjola <juha.yrjola@solidboot.com>
Cc: Steven J. Hill <Steven.Hill@cavium.com>
Cc: Shawn Lin <shawn.lin@rock-chips.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Suggested-by: Steven J. Hill <Steven.Hill@cavium.com>
Suggested-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
2017-10-04 10:22:55 +02:00
..
card.h mmc: core: correct taac parameter according to the specification 2017-08-30 14:01:46 +02:00
core.h mmc: core: Move mmc_start_areq() declaration 2017-08-30 15:03:53 +02:00
host.h mmc: Delete bounce buffer handling 2017-10-04 10:22:55 +02:00
mmc.h mmc: Adding AUTO_BKOPS_EN bit set for Auto BKOPS support 2017-02-14 09:11:00 +01:00
pm.h mmc: Standardize header file inclusion checks. 2011-07-20 17:20:48 -04:00
sd.h mmc: Standardize header file inclusion checks. 2011-07-20 17:20:48 -04:00
sdhci-pci-data.h mmc: sdhci-pci: Add support for drive strength selection for SPT 2015-06-01 09:07:14 +02:00
sdio.h mmc: sdio: fix setting card data bus width as 4-bit 2012-06-06 09:22:53 -04:00
sdio_func.h mmc: sdio: fix alignment issue in struct sdio_func 2017-04-18 19:18:07 +02:00
sdio_ids.h brcmfmac: Add support for CYW4373 SDIO/USB chipset 2017-08-08 14:51:12 +03:00
sh_mmcif.h mmc: sh_mmcif: Remove unused clk_ctrl2_present from the platform data 2017-02-13 13:19:58 +01:00
slot-gpio.h mmc: core: First step in cleaning up public mmc header files 2017-02-13 13:20:19 +01:00