mmc: jz4740: Work around bug on JZ4760(B)

On JZ4760 and JZ4760B, SD cards fail to run if the maximum clock
rate is set to 50 MHz, even though the controller officially does
support it.

Until the actual bug is found and fixed, limit the maximum clock rate to
24 MHz.

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

Author: pcercuei

drivers/mmc/host/jz4740_mmc.c

@@ -1053,6 +1053,16 @@ static int jz4740_mmc_probe(struct platform_device* pdev)
 	mmc->ops = &jz4740_mmc_ops;
 	if (!mmc->f_max)
 		mmc->f_max = JZ_MMC_CLK_RATE;
+
+	/*
+	 * There seems to be a problem with this driver on the JZ4760 and
+	 * JZ4760B SoCs. There, when using the maximum rate supported (50 MHz),
+	 * the communication fails with many SD cards.
+	 * Until this bug is sorted out, limit the maximum rate to 24 MHz.
+	 */
+	if (host->version == JZ_MMC_JZ4760 && mmc->f_max > JZ_MMC_CLK_RATE)
+		mmc->f_max = JZ_MMC_CLK_RATE;
+
 	mmc->f_min = mmc->f_max / 128;
 	mmc->ocr_avail = MMC_VDD_32_33 | MMC_VDD_33_34;