[MMC] Explain the internals of mmc_power_up()
It seems that people get confused about what is happening in
mmc_power_up(). Add a comment to make it clear why we have
a two stage process.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index b586a83..eb41391 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -679,7 +679,15 @@
}
/*
- * Apply power to the MMC stack.
+ * Apply power to the MMC stack. This is a two-stage process.
+ * First, we enable power to the card without the clock running.
+ * We then wait a bit for the power to stabilise. Finally,
+ * enable the bus drivers and clock to the card.
+ *
+ * We must _NOT_ enable the clock prior to power stablising.
+ *
+ * If a host does all the power sequencing itself, ignore the
+ * initial MMC_POWER_UP stage.
*/
static void mmc_power_up(struct mmc_host *host)
{