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pinctrl/nomadik: document Alt-C glitch

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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>
This commit is contained in:
Linus Walleij 2012-05-22 11:46:45 +02:00
parent 14e1e9f5ca
commit daf731748f
1 changed files with 21 additions and 1 deletions
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22
drivers/pinctrl/pinctrl-nomadik.c
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@ -1438,7 +1438,27 @@ static int nmk_pmx_enable(struct pinctrl_dev *pctldev, unsigned function,
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) {