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

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

+Cc: Bart (as GPIOLIB maintainer and author of the reset-gpio rework).

On Mon, Oct 19, 2026 at 07:03:57AM -0400, Michael Anderson wrote:
> 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?

But then it sounds like the request-by-demand?
Bart, any ideas how to address this case?

> On Mon, Oct 19, 2026 at 5:58 AM Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> 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