sdhci: describe quirks

Add a comment for each quirk to describe what it does and why.
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>

drivers/mmc/host/sdhci.c

@@ -26,12 +26,22 @@
 
 static unsigned int debug_quirks = 0;
 
+/*
+ * Different quirks to handle when the hardware deviates from a strict
+ * interpretation of the SDHCI specification.
+ */
+
+/* Controller doesn't honor resets unless we touch the clock register */
 #define SDHCI_QUIRK_CLOCK_BEFORE_RESET			(1<<0)
+/* Controller has bad caps bits, but really supports DMA */
 #define SDHCI_QUIRK_FORCE_DMA				(1<<1)
 /* Controller doesn't like some resets when there is no card inserted. */
 #define SDHCI_QUIRK_NO_CARD_NO_RESET			(1<<2)
+/* Controller doesn't like clearing the power reg before a change */
 #define SDHCI_QUIRK_SINGLE_POWER_WRITE			(1<<3)
+/* Controller has flaky internal state so reset it on each ios change */
 #define SDHCI_QUIRK_RESET_CMD_DATA_ON_IOS		(1<<4)
+/* Controller has an unusable DMA engine */
 #define SDHCI_QUIRK_BROKEN_DMA				(1<<5)
 
 static const struct pci_device_id pci_ids[] __devinitdata = {