GPIOs not correctly exported via sysfs on ATSAMA5D2

[Romain Izard <romain.izard.pro@xxxxxxxxx>]

Hello,

While experimenting with a new chip, I connected it on the SDIO
interface on my board based on a SAMA5D2 SoC. For a first step, I need
to drive the pins on the SDIO bus at a given level to program this new
chip. To do so, I tried to control the GPIO lines manually by unbinding
the SDHCI controller, and using /sys/class/gpio/export to control the
pins, with the following code:

echo a0000000.sdio-host > /sys/bus/platform/drivers/sdhci-at91/unbind
echo 4 > /sys/class/gpio/export
echo low > /sys/class/gpio/PA4/direction

Unfortunately, the state of the pin does not change and it remains
driven to 1. I checked the configuration register with devmem2, and it
appeared that the selected function remains the SDIO function even after
calling export.

The issue does not appear when I use a GPIO in a driver with an explicit
pinctrl configuration in the device tree, which explains why I did not
see it until now.

The kernel version used is Linux 5.4.22

Is this a user error from my part, or is there something missing in the
AT91 PIO4 pinctrl driver ?

Best regards,
-- 
Romain Izard