Re: [PATCH v2] pinctrl: baytrail: Fix pin being driven low for a while on gpiod_get(..., GPIOD_OUT_HIGH)

[Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>]

On Sat, Jun 06, 2020 at 11:31:50AM +0200, Hans de Goede wrote:
> The pins on the Bay Trail SoC have separate input-buffer and output-buffer
> enable bits and a read of the level bit of the value register will always
> return the value from the input-buffer.
> 
> The BIOS of a device may configure a pin in output-only mode, only enabling
> the output buffer, and write 1 to the level bit to drive the pin high.
> This 1 written to the level bit will be stored inside the data-latch of the
> output buffer.
> 
> But a subsequent read of the value register will return 0 for the level bit
> because the input-buffer is disabled. This causes a read-modify-write as
> done by byt_gpio_set_direction() to write 0 to the level bit, driving the
> pin low!
> 
> Before this commit byt_gpio_direction_output() relied on
> pinctrl_gpio_direction_output() to set the direction, followed by a call
> to byt_gpio_set() to apply the selected value. This causes the pin to
> go low between the pinctrl_gpio_direction_output() and byt_gpio_set()
> calls.
> 
> Change byt_gpio_direction_output() to directly make the register
> modifications itself instead. Replacing the 2 subsequent writes to the
> value register with a single write.
> 
> Note that the pinctrl code does not keep track internally of the direction,
> so not going through pinctrl_gpio_direction_output() is not an issue.
> 
> This issue was noticed on a Trekstor SurfTab Twin 10.1. When the panel is
> already on at boot (no external monitor connected), then the i915 driver
> does a gpiod_get(..., GPIOD_OUT_HIGH) for the panel-enable GPIO. The
> temporarily going low of that GPIO was causing the panel to reset itself
> after which it would not show an image until it was turned off and back on
> again (until a full modeset was done on it). This commit fixes this.
> 
> This commit also updates the byt_gpio_direction_input() to use direct
> register accesses instead of going through pinctrl_gpio_direction_input(),
> to keep it consistent with byt_gpio_direction_output().
> 
> Note for backporting, this commit depends on:
> commit e2b74419e5cc ("pinctrl: baytrail: Replace WARN with dev_info_once
> when setting direct-irq pin to output")
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 86e3ef812fe3 ("pinctrl: baytrail: Update gpio chip operations")
> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>

Acked-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>