Re: [PATCH] drm/i915/dsi: Use owned panel/backlight GPIOs in VLV MIPI sequences

[Michael Anderson]

Thanks for looking at this. I'd like to make sure I understand before
doing a v2.

Here both users of pin 10 are i915 itself: intel_dsi_vbt_gpio_init()
takes it as the "backlight" enable GPIO (soc_panel_gpio_table,
PPS_BLC_SOC), and the VBT MIPI sequences on this tablet then toggle the
same pin as the panel reset (low in ASSERT_RESET, high-low-high in
DEASSERT_RESET). Do you mean describing pin 10 with a software-node
"reset-gpios" reference and having both paths go through
reset_control_*(), or something else?

One concern: the VBT drives explicit pulses on the pin, and shared reset
controls are refcounted, so a pulse from one path wouldn't reach the line
while the other holds it deasserted. Would an exclusive reset obtained
once by i915 and used by both paths work instead, or am I missing how
reset-gpio handles this?

Thanks,
Mike


On Mon, Oct 19, 2026 at 5:58 AM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Sun, Oct 18, 2026 at 04:12:40PM -0400, Mike Anderson via B4 Relay wrote:
>
> > On VLV with PPS_BLC_SOC, intel_dsi_vbt_gpio_init() requests the
> > backlight and panel enable GPIOs (INT33FC:01 pins 10 and 11) for i915
> > itself. Since commit 8241b55f1ded ("drm/i915/dsi: Replace poking of VLV
> > GPIOs behind the driver's back") the GPIO elements of the VBT MIPI
> > sequences also request their pin through gpiolib, so when a sequence
> > toggles one of these two pins the request fails and the toggle is
> > dropped:
> >
> >   i915 0000:00:02.0: [drm] *ERROR* GPIO index 10 request failed (-EBUSY)
> >
> > Before that commit the pad registers were written directly, so the
> > sequences could still drive the pins i915 owned.
> >
> > The Toshiba Encore 2 WT8-B (Bay Trail, DSI panel) uses pin 10 as the
> > panel reset line: MIPI_SEQ_ASSERT_RESET drives it low and
> > MIPI_SEQ_DEASSERT_RESET pulses it high-low-high. With every toggle
> > failing, the panel is never reset after i915 sends MIPI_SEQ_DISPLAY_OFF
> > during takeover and stays black, even though the backlight PWM and both
> > enable GPIOs read back as on. The tablet only has a usable display
> > with nomodeset.
> >
> > When a VLV MIPI sequence targets pin 10 or 11 and i915 holds the
> > corresponding descriptor, set it through that descriptor instead of
> > requesting the pin a second time.
>
> Ah, nice, this should be solved via using reset-gpio, which is exactly fixed
> for the reset shared lines case.
>
> --
> With Best Regards,
> Andy Shevchenko
>
>


-- 
Michael Anderson