[RFC] i.MX25/35/SDHCI: switch off DMA usage

[Juergen Borleis]

DMA and the required overhead on very small data blocks seems an expensive
operation. Due to erratum ENGCM07207 for i.MX25 and i.MX35 SoCs the
support for multiblock transfers is disabled which results into a huge
amount of single 512 byte sector transfers and interrupts. This slows down
the transmission speed to below 500 kiB/s (even at 50 MHz SD card clock).
Using PIO instead of DMA to avoid ENGCM07207 happens and re-enabling
multiblock transfers again improve the transmission capability up to about
2.5 MiB/s.

I'm still not sure if ENGCM07207 is related to DMA only and can not happen
when PIO is used instead. Someone out there with experience regarding this
topic?
---
 drivers/mmc/host/sdhci-esdhc-imx.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/mmc/host/sdhci-esdhc-imx.c b/drivers/mmc/host/sdhci-esdhc-imx.c
index 10ef8244a239..f5fd569a17c3 100644
--- a/drivers/mmc/host/sdhci-esdhc-imx.c
+++ b/drivers/mmc/host/sdhci-esdhc-imx.c
@@ -976,8 +976,8 @@ static int sdhci_esdhc_imx_probe(struct platform_device *pdev)
 
 	if (imx_data->socdata->flags & ESDHC_FLAG_ENGCM07207)
 		/* Fix errata ENGcm07207 present on i.MX25 and i.MX35 */
-		host->quirks |= SDHCI_QUIRK_NO_MULTIBLOCK
-			| SDHCI_QUIRK_BROKEN_ADMA;
+		host->quirks |=
+			SDHCI_QUIRK_BROKEN_ADMA | SDHCI_QUIRK_BROKEN_DMA;
 
 	/*
 	 * The imx6q ROM code will change the default watermark level setting
-- 
1.7.10.4

Juergen
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