[PATCH v2] mmc: sdhci: Use Auto CMD Auto Select only when v4_mode is true

[Jisheng Zhang <Jisheng.Zhang@synaptics.com>]

sdhci-of-dwcmshc meets an eMMC read performance regression with below
command after commit 427b6514d095 ("mmc: sdhci: Add Auto CMD Auto
Select support"):

dd if=/dev/mmcblk0 of=/dev/null bs=8192 count=100000

Before the commit, the above command gives 120MB/s
After the commit, the above command gives 51.3 MB/s

So it looks like sdhci-of-dwcmshc expects Version 4 Mode for Auto
CMD Auto Select. Fix the performance degradation by ensuring v4_mode
is true to use Auto CMD Auto Select.

Fixes: 427b6514d095 ("mmc: sdhci: Add Auto CMD Auto Select support")
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
---
Since v1:
  - add the performance degradation on sdhci-of-dwcmshc explanation in
    commit msg
  - add a comment in the code explaining we require Version 4 Mode because some
    controllers(e.g sdhci-of-dwcmshc) expect it that way.

 drivers/mmc/host/sdhci.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
index 592a55a34b58..3561ae8a481a 100644
--- a/drivers/mmc/host/sdhci.c
+++ b/drivers/mmc/host/sdhci.c
@@ -1384,9 +1384,11 @@ static inline void sdhci_auto_cmd_select(struct sdhci_host *host,
 	/*
 	 * In case of Version 4.10 or later, use of 'Auto CMD Auto
 	 * Select' is recommended rather than use of 'Auto CMD12
-	 * Enable' or 'Auto CMD23 Enable'.
+	 * Enable' or 'Auto CMD23 Enable'. We require Version 4 Mode
+	 * here because some controllers (e.g sdhci-of-dwmshc) expect it.
 	 */
-	if (host->version >= SDHCI_SPEC_410 && (use_cmd12 || use_cmd23)) {
+	if (host->version >= SDHCI_SPEC_410 && host->v4_mode &&
+	    (use_cmd12 || use_cmd23)) {
 		*mode |= SDHCI_TRNS_AUTO_SEL;
 
 		ctrl2 = sdhci_readw(host, SDHCI_HOST_CONTROL2);
-- 
2.28.0