MMC: Fix handling of low-voltage cards

Fix handling of low voltage MMC cards.

The latest MMC and SD specs both agree that support for
low-voltage operations is indicated by bit 7 in the OCR.
The MMC spec states that the low voltage range is
1.65-1.95V while the SD spec leaves the actual voltage
range undefined - meaning that there is still no such
thing as a low voltage SD card.

However, an old Sandisk spec implied that bits 7.0
represented voltages below 2.0V in 1V or 0.5V increments,
and the code was accordingly written with that expectation.

This confusion meant that host drivers attempting to support
the typical low voltage (1.8V) would set the wrong bits in
the host OCR mask (usually bits 5 and/or 6) resulting in the
the low voltage mode never being used.

This change corrects the low voltage range and adds sanity
checks on the reserved bits (0-6) and for SD cards that
claim to support low-voltage operations.

Signed-off-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>

2 files changed, 29 insertions, 0 deletions

diff --git a/drivers/mmc/core/mmc.c b/drivers/mmc/core/mmc.c
index c528017d6128..c2e120b11bef 100644
--- a/drivers/mmc/core/mmc.c
+++ b/drivers/mmc/core/mmc.c
@@ -323,6 +323,17 @@ int mmc_attach_mmc(struct mmc_host *host, u32 ocr)
 
 	mmc_attach_bus(host, &mmc_ops);
 
+	/*
+	 * Sanity check the voltages that the card claims to
+	 * support.
+	 */
+	if (ocr & 0x7F) {
+		printk(KERN_WARNING "%s: card claims to support voltages "
+		       "below the defined range. These will be ignored.\n",
+		       mmc_hostname(host));
+		ocr &= ~0x7F;
+	}
+
 	host->ocr = mmc_select_voltage(host, ocr);
 
 	/*
diff --git a/drivers/mmc/core/sd.c b/drivers/mmc/core/sd.c
index 6c6beb48f3a8..fb18b301502e 100644
--- a/drivers/mmc/core/sd.c
+++ b/drivers/mmc/core/sd.c
@@ -297,6 +297,24 @@ int mmc_attach_sd(struct mmc_host *host, u32 ocr)
 
 	mmc_attach_bus(host, &mmc_sd_ops);
 
+	/*
+	 * Sanity check the voltages that the card claims to
+	 * support.
+	 */
+	if (ocr & 0x7F) {
+		printk(KERN_WARNING "%s: card claims to support voltages "
+		       "below the defined range. These will be ignored.\n",
+		       mmc_hostname(host));
+		ocr &= ~0x7F;
+	}
+
+	if (ocr & MMC_VDD_165_195) {
+		printk(KERN_WARNING "%s: SD card claims to support the "
+		       "incompletely defined 'low voltage range'. This "
+		       "will be ignored.\n", mmc_hostname(host));
+		ocr &= ~MMC_VDD_165_195;
+	}
+
 	host->ocr = mmc_select_voltage(host, ocr);
 
 	/*