[RFC] mmc: meson-gx-mmc: add delay during poweroff

[Da Xue]

On Wed, Jul 2, 2025 at 4:57 PM Martin Blumenstingl
<martin.blumenstingl at googlemail.com> wrote:
>
> On Wed, Jul 2, 2025 at 9:07 PM Da Xue <da at libre.computer> wrote:
> >
> > On Wed, Jul 2, 2025 at 2:40 PM Martin Blumenstingl
> > <martin.blumenstingl at googlemail.com> wrote:
> > >
> > > On Wed, Jul 2, 2025 at 7:22 PM Da Xue <da at libre.computer> wrote:
> > > >
> > > > On Wed, Jul 2, 2025 at 1:07 PM Jerome Brunet <jbrunet at baylibre.com> wrote:
> > > > ...
> > > > > If, as the description suggest, the regulator framework somehow ignore
> > > > > the timing set in DT, maybe this is what needs to be checked ?
> > > >
> > > > The regulator framework only cares about timing for regulator on.
> > > > Regulator off just turns off the regulator and returns without delay.
> > > There's an exception to this: gpio-regulators without an enable-gpios
> > > property. My understanding is that regulator_disable() is a no-op in
> > > that case (meson_mmc_set_ios() even has a comment above the
> > > switch/case statement), see [0].
> > >
> > > > The code makes incorrect assumptions. Then the kernel resets the board
> > > > without having enough time.
> > > Can you please name the board you're testing? I'm worried that I'll be
> > > looking at one .dts but you're looking at another one.
> >
> > https://github.com/libre-computer-project/libretech-linux/blob/master/arch/arm64/boot/dts/amlogic/meson-libretech-cottonwood.dtsi#L481
> >
> > vcc_card is a gpio regulator that gets toggled on->off->on.
> Thanks, that clears things up as I was indeed looking at a gpio
> regulator while this is a fixed regulator!
>
> > I traced the regulator framework a few weeks ago and forgot the final
> > regulator disable function call, but that call basically returned
> > immediately while the regulator-enable function complement had delays
> > implemented.
> Yep, for fixed regulators there's an "off-on-delay-us" device-tree
> property (which translates to "off_on_delay" in the code).
> Its implementation is smart enough to not waste time by adding delays
> at runtime by implementing: on -> off + remember time -> wait
> remaining time + on (meaning: if there was enough time between off and
> the second on there's no additional wait) [0]. On system shutdown it
> will not add any delay unfortunately (where Linux loses control over
> time-keeping), meaning we can end up with too little waiting time.

Yes, this is evident on quite a few Amlogic boards but occurred rarely
enough that it can be overlooked but never-the-less should be
addressed.

On our SM1 board, this occurs more often than not. With this patch, we
can reboot the loop indefinitely.

>
> Also my understanding is that it's not something that can be fixed in
> u-boot or TF-A. This is because bootrom already has trouble reading
> the next stage from an SD card (which is a valid boot media).

Correct, not fixable in TF-A or u-boot.

>
>
> [0] https://elixir.bootlin.com/linux/v6.15/source/drivers/regulator/core.c#L2754