mmc: sdhci-of-esdhc: fix transfer mode register reading

The standard SD controller uses two 16-bit registers for
command sending.
0xC: Transfer Mode Register
0xE: Command Register

But the eSDHC controller uses one 32-bit register instead.
0xC: XFERTYPE

For Transfer Mode Register and Command Register writing,
the eSDHC driver will store Transfer Mode Register value in
a variable first. When Command Register writing happens,
driver will directly write a 32-bit value into XFERTYPE
register.

But for Transfer Mode Register reading, driver just returns
a actual value. This may cause issue for some read-modify-write
operations. We should make both reading and write on that variable
for Transfer Mode Register.

Signed-off-by: Yangbo Lu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Ulf Hansson <[email protected]>

Author: yangbolu1991

drivers/mmc/host/sdhci-of-esdhc.c

@@ -173,6 +173,9 @@ static u16 esdhc_readw_fixup(struct sdhci_host *host,
 	u16 ret;
 	int shift = (spec_reg & 0x2) * 8;
 
+	if (spec_reg == SDHCI_TRANSFER_MODE)
+		return pltfm_host->xfer_mode_shadow;
+
 	if (spec_reg == SDHCI_HOST_VERSION)
 		ret = value & 0xffff;
 	else