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

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

On Fri, Jun 05, 2020 at 04:33:47PM +0200, Hans de Goede wrote:
> On 6/2/20 5:23 PM, Andy Shevchenko wrote:
> > On Tue, Jun 02, 2020 at 02:21:30PM +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.
> > 
> > No Fixes tag?
> 
> It is sort of hard to pin the introduction of this down to a single
> commit. If I were to guess, I guess the commit introducing the driver?

Why not? Good guess to me (but I think rather the one which converts GPIO
driver to pin control).

...

> > > +	/*
> > > +	 * Before making any direction modifications, do a check if gpio is set
> > 
> > > +	 * for direct IRQ.  On baytrail, setting GPIO to output does not make
> > 
> > Since we change this, perhaps
> > 
> > 'IRQ.  On baytrail' -> 'IRQ. On Baytrail' (one space and capital 'B').
> 
> Sure, not sure if that is worth respinning the patch for though,
> either way let me know.

I think makes sense to respin. We still have time.

> > > +	 * sense, so let's at least inform the caller before they shoot
> > > +	 * themselves in the foot.
> > > +	 */

...

> > Wouldn't be simple below fix the issue?

> No that will not help the pin is already high, but any reads
> of the register will return the BYT_LEVEL bit as being low, so
> the read-write-modify done when setting the direction reads BYT_LEVEL
> as 0 and writes it back as such.

So, if I read documentation correctly, there is no means to read back current
output value if input is disabled. Alas, quite a bad design of hardware.
And on top of that likely nobody has tested that on non-Windows platform.

> So your proposal would actually make the problem much worse (and more
> obvious) if we do the byt_gpio_set() first then for pins which have
> there input-buffer initially disabled, the value passed to
> byt_gpio_direction_output will be completely ignored and they will
> always end up as being driven low.

What I proposed is not gonna work AFAIU documentation.

Btw, can we for sake of consistency update direction_input() as well?

-- 
With Best Regards,
Andy Shevchenko