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
