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

[Martin Blumenstingl]

Hi Anand,

On Tue, Jul 1, 2025 at 12:00 PM Anand Moon <linux.amoon at gmail.com> wrote:
>
> Hi Da,
>
> On Sat, 28 Jun 2025 at 09:15, Da Xue <da at libre.computer> wrote:
> >
> > Regulators controlling the SD card power need some settling time for SD
> > cards to fully reset from UHS modes. The regulator framework seems to
> > ignore falling times set in the device tree causing a few boards with the
> > same hardware implementation to hang on reboot because the SD card still
> > had some voltage and did not reset properly to be initialized again.
> >
> > Add a delay sufficiently long for the voltage to drop so that the SD card
> > can reset properly. Otherwise the reboot will hang at missing SD card
> > especially with Samsung cards.
> >
> Although the driver defines reset identifiers such as
> RESET_SD_EMMC_A, RESET_SD_EMMC_B, and RESET_SD_EMMC_C,
> It does not implement proper reset controller functionality,
> specifically lacking support
> for reset_control_assert() and reset_control_deassert() operations.
I think there's a misunderstanding:
The meson-gx-mmc driver calls device_reset_optional() during .probe
which will internally call reset_control_reset().
So I don't see a problem here.

The patch seems more about power sequencing, where either the SD card
or regulator used to power the SD card requires a certain amount of
time (delay) when switching from ON -> OFF -> ON (my understanding is:
without this delay the card sees ON -> ON which fails to update some
state internally).

To me it's not clear if this is a property of the SD spec or rather
the regulator.
Ulf, Jerome - any ideas / inputs from you?


Best regards,
Martin