[PATCH] mmc: core: properly select voltage range without power cycle

[Yann Gautier]

In mmc_select_voltage(), if there is no full power cycle, the voltage
range selected at the end of the function will be on a single range
(e.g. 3.3V/3.4V). To keep a range around the selected voltage (3.2V/3.4V),
the mask shift should be reduced by 1.

This issue was triggered by using a specific SD-card (Verbatim Premium
16GB UHS-1) on an STM32MP157C-DK2 board. This board cannot do UHS modes
and there is no power cycle. And the card was failing to switch to
high-speed mode. When adding the range 3.2V/3.3V for this card with the
proposed shift change, the card can switch to high-speed mode.

Fixes: ce69d37b7d8f ("mmc: core: Prevent violation of specs while initializing cards")
Signed-off-by: Yann Gautier <yann.gautier@xxxxxxxxxxx>
---
 drivers/mmc/core/core.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index 95fa8fb1d45f..c5de202f530a 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -1134,7 +1134,13 @@ u32 mmc_select_voltage(struct mmc_host *host, u32 ocr)
 		mmc_power_cycle(host, ocr);
 	} else {
 		bit = fls(ocr) - 1;
-		ocr &= 3 << bit;
+		/*
+		 * The bit variable represents the highest voltage bit set in
+		 * the OCR register.
+		 * To keep a range of 2 values (e.g. 3.2V/3.3V and 3.3V/3.4V),
+		 * we must shift the mask '3' with (bit - 1).
+		 */
+		ocr &= 3 << (bit - 1);
 		if (bit != host->ios.vdd)
 			dev_warn(mmc_dev(host), "exceeding card's volts\n");
 	}
-- 
2.25.1