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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Mon, Jun 08, 2020 at 01:59:53PM +0300, Mika Westerberg wrote:
> 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@vger.kernel.org
> > Fixes: 86e3ef812fe3 ("pinctrl: baytrail: Update gpio chip operations")
> > Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> 
> Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>

Pushed to my review and testing queue, thanks!

-- 
With Best Regards,
Andy Shevchenko