Am Fri, 3 Mar 2017 10:08:41 +0100 hat Matthijs van Duin
<matthijsvanduin@xxxxxxxxx> geschrieben:
Interesting find.
Could that have something to do with the weird hangs while booting?
Depending on the power usage of the SD Card, it could be that the SD
Card you're booting from simply freezes because it doesn't get enough
power... so instability might be caused by that.
Is that something we need to fix with software or does the hardware
need to be changed as well.
> Like its predecessors, the omap5 has a special bank of I/Os for
> interfacing with sdcards, which (unlike other I/O) may operate at 3V
> in addition to 1.8V, with support for switching at runtime.
>
> Unfortunately the designated LDO for this supply in the PMIC
> (Palmas), ldo9, can only supply 50 mA while cards are known to
> require more than that in practice. The omap5-uevm solved this by
> using a separate 3.3v supply for the sdcard, but disturbingly they
> continued to use ldo9 for the I/O supply on the omap5...
>
> The device tree right now permits ldo9 to be lowered to 1.8V, unaware
> of the fact that the sdcard will continue to drive 3.3V signals into
> the now very unhappy I/Os of the omap5 !
>
> Hence, the most urgent fix would be:
>
> &ldo9_reg {
> regulator-min-microvolt = <3000000>;
> };
>
>
> To make things worse, according to the Recommended Operating
> Conditions in the omap5 datasheet, vdds_sdcard should not be operated
> at 3.3V at all but 3.0V nominal (3.06V MAX DC).
>
> This means you have to choose between:
> - configure ldo9 in bypass. Both sides use 3.3v, but this exceeds the
> max value specified by the omap5 datasheet.
> - configure ldo9 at 3.0V. The sdcard will be driving signals 0.3v
> higher than the omap5's supply (+ overshoot, if any).
>
> Pick your poison.
>
> Afaik bypass is the power-up config, while u-boot changes it to 3.0V.
> I'm not sure what linux does currently since omap5-board-common.dtsi
> incorrectly claims that &vmmcsd_fixed is 3.0V instead of 3.3V.
>
> I'm guessing that ldo9 at 3.0V is indeed the intended modus operandi.
>
> (side-note: ldo8 can provide 200 mA, which would have been more than
> enough, and is not connected to anything whatsoever... nicely done)
>
>
> A final problem I noticed is power switching. The DT as-is implies
> that the sdcard can be powered off by switching ldo9 but this is
> plain wrong. Powering down the sdcard should be done by switching
> &vmmcsd_fixed via REGEN3. An obstacle is that eMMC (&mmc2) uses it
> as supply, but fortunately DT is simply wrong: eMMC is powered
> directly from the main 3.3v supply.
>
> So, roughly:
>
> vcc_3v3_main: fixedregulator-vcc_3v3_main {
> compatible = "regulator-fixed";
> regulator-name = "vcc_3v3_main";
> vin-supply = <&vmain>;
> regulator-min-microvolt = <3300000>;
> regulator-max-microvolt = <3300000>;
> };
>
> &vmmcsd_fixed {
> /* called vcc_3v3_sdio on omap5-uevm */
> vin-supply = <&vcc_3v3_main>;
> regulator-min-microvolt = <3300000>;
> regulator-max-microvolt = <3300000>;
> gpio = /* FIXME palmas REGEN3 */;
> };
>
> &mmc1 {
> /* card supply is actually &vmmcsd_fixed */
> vmmc-supply = <&ldo9_reg>;
> };
>
> &mmc2 {
> vmmc-supply = <&vcc_3v3_main>;
> };
>
> The details remaining are how to actually control REGEN3, and how to
> make sure linux leaves ldo9 alone and switches the supply just using
> &vmmcsd_fixed. I'm hoping someone who understands this stuff better
> than me has a suggestion?
>
>
> Matthijs
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