pinctrl/nomadik: document Alt-C glitch
This documentation comment existed in an earlier patch set for
GPIO consolidation, so I'm saving it for maintainability of the
code.
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/pinctrl/pinctrl-nomadik.c b/drivers/pinctrl/pinctrl-nomadik.c
index e8937e7..3e7e47d 100644
--- a/drivers/pinctrl/pinctrl-nomadik.c
+++ b/drivers/pinctrl/pinctrl-nomadik.c
@@ -1438,7 +1438,27 @@
dev_dbg(npct->dev, "enable group %s, %u pins\n", g->name, g->npins);
- /* Handle this special glitch on altfunction C */
+ /*
+ * If we're setting altfunc C by setting both AFSLA and AFSLB to 1,
+ * we may pass through an undesired state. In this case we take
+ * some extra care.
+ *
+ * Safe sequence used to switch IOs between GPIO and Alternate-C mode:
+ * - Save SLPM registers (since we have a shadow register in the
+ * nmk_chip we're using that as backup)
+ * - Set SLPM=0 for the IOs you want to switch and others to 1
+ * - Configure the GPIO registers for the IOs that are being switched
+ * - Set IOFORCE=1
+ * - Modify the AFLSA/B registers for the IOs that are being switched
+ * - Set IOFORCE=0
+ * - Restore SLPM registers
+ * - Any spurious wake up event during switch sequence to be ignored
+ * and cleared
+ *
+ * We REALLY need to save ALL slpm registers, because the external
+ * IOFORCE will switch *all* ports to their sleepmode setting to as
+ * to avoid glitches. (Not just one port!)
+ */
glitch = (g->altsetting == NMK_GPIO_ALT_C);
if (glitch) {