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authorLinus Walleij <linus.walleij@linaro.org>2017-09-20 10:56:14 +0200
committerUlf Hansson <ulf.hansson@linaro.org>2017-10-04 10:22:55 +0200
commitde3ee99b097dd51938276e3af388cd4ad0f2750a (patch)
treeedd6f7892e67f61f8e8a26a15d3a50bad1d975dc /include
parentfb458864d9a78cc433fec7979acbe4078c82d7a8 (diff)
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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 a45c6cb81647 ("[ARM] 5369/1: omap mmc: Add new omap hsmmc controller for 2430 and 34xx, v3") This optimization was removed in commit 0ccd76d4c236 ("omap_hsmmc: Implement scatter-gather emulation") which found that scatter-gather emulation provided even better performance. The same was introduced for SDHCI in commit 2134a922c6e7 ("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>
Diffstat (limited to 'include')
-rw-r--r--include/linux/mmc/host.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index f3f2d07feb2a..9a43763a68ad 100644
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -316,7 +316,7 @@ 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 */
+/* (1 << 21) is free for reuse */
#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 */