[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 <[email protected]>
chewitt · Dec 14, 2005 · 45f8245 · 45f8245
1 parent cd61045
commit 45f8245
Showing 1 changed file with 9 additions and 1 deletion.
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
@@ -679,7 +679,15 @@ static void mmc_idle_cards(struct mmc_host *host)
 }
 
 /*
- * 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)
 {