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

[Martin Blumenstingl]

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.

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).


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