Re: [PATCH] i915: Introduce quirk for shifting eDP brightness.

[Kevin Chowski <chowski@chromium.org>]

Apologies for being too vague. To be as precise I can be, here is the
specific code delta I tested: https://crrev.com/c/2406616 . To answer
your other question, the code I tested against is indeed including the
fde7266fb2f6 (despite ostensibly being called 5.4 in my commit
message): our current top-of-tree for our 5.4 branch includes the
intel_dp_aux_calc_max_backlight logic. Further, I'll note that change
is exactly the change which breaks my Pixelbook model: prior to the
change, the max_brightness was hard-coded to 0xFFFF and the math
worked out that it didn't matter that the hardware cared about the MSB
despite the driver code caring about the LSB.

To answer Ville's question: the fde7266fb2f6 change which fixes one
laptop (I believe Thinkpad X1 extreme Gen 2, from some bug reports I
dug up) and breaks another (Pixelbook); so unfortunately I believe we
need a quirk at least for some laptop. Reading through the copy of the
datasheet I have, it wasn't clear to me which was the correct
interpretation. I'm cc'ing puthik@, who was leaning toward the current
kernel code (caring about LSB) being the correct interpretation. I
believe we have other chromebooks which do rely on LSB functionality,
so unless we can find more examples of laptops wanting MSB it
currently looks like Pixelbook is the outlier.

On Thu, Sep 17, 2020 at 11:28 AM Jani Nikula
<jani.nikula@linux.intel.com> wrote:
>
> On Thu, 17 Sep 2020, Kevin Chowski <chowski@chromium.org> wrote:
> > We have observed that Google Pixelbook's backlight hardware is
> > interpretting these backlight bits from the most-significant side of the
> > 16 bit word (if DP_EDP_PWMGEN_BIT_COUNT < 16), whereas the driver code
> > assumes the peripheral cares about the least-significant bits.
> >
> > Testing was done from within Chrome OS's build environment when the
> > patch is backported to 5.4 (the version we are newly targeting for the
> > Pixelbook); for the record:
> >    $ emerge-eve-kernelnext sys-kernel/chromeos-kernel-5_4 && \
> >       ./update_kernel.sh --remote=$IP
> >
> > I used `/sys/kernel/debug/dri/0/eDP-1/i915_dpcd` on my laptop to verify
> > that the registers were being set according to what the actual hardware
> > expects; I also observe that the backlight is noticeably brighter with
> > this patch.
>
> It's unclear to me what kernel version this is against, and what you've
> actually tested.
>
> Have you tried v5.7 kernel with Lyude's fde7266fb2f6 ("drm/i915: Fix eDP
> DPCD aux max backlight calculations")?
>
> I just want to make sure you've tested with all the relevant fixes
> before adding quirks.
>
> BR,
> Jani.
>
> >
> > Signed-off-by: Kevin Chowski <chowski@chromium.org>
> > ---
> >
> >  .../drm/i915/display/intel_dp_aux_backlight.c | 34 +++++++++++++++++++
> >  drivers/gpu/drm/i915/display/intel_quirks.c   | 13 +++++++
> >  drivers/gpu/drm/i915/i915_drv.h               |  1 +
> >  3 files changed, 48 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > index acbd7eb66cbe3..99c98f217356d 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > @@ -91,6 +91,23 @@ static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
> >       if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> >               level = (read_val[0] << 8 | read_val[1]);
> >
> > +     if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
> > +             if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
> > +                                             &read_val[0])) {
> > +                     DRM_DEBUG_KMS("Failed to read DPCD register 0x%x\n",
> > +                                     DP_EDP_PWMGEN_BIT_COUNT);
> > +                     return 0;
> > +             }
> > +             // Only bits 4:0 are used, 7:5 are reserved.
> > +             read_val[0] = read_val[0] & 0x1F;
> > +             if (read_val[0] > 16) {
> > +                     DRM_DEBUG_KMS("Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
> > +                                             read_val[0]);
> > +                     return 0;
> > +             }
> > +             level >>= 16 - read_val[0];
> > +     }
> > +
> >       return level;
> >  }
> >
> > @@ -106,6 +123,23 @@ intel_dp_aux_set_backlight(const struct drm_connector_state *conn_state, u32 lev
> >       struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> >       u8 vals[2] = { 0x0 };
> >
> > +     if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
> > +             if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
> > +                                             &vals[0])) {
> > +                     DRM_DEBUG_KMS("Failed to write aux backlight level: Failed to read DPCD register 0x%x\n",
> > +                                       DP_EDP_PWMGEN_BIT_COUNT);
> > +                     return;
> > +             }
> > +             // Only bits 4:0 are used, 7:5 are reserved.
> > +             vals[0] = vals[0] & 0x1F;
> > +             if (vals[0] > 16) {
> > +                     DRM_DEBUG_KMS("Failed to write aux backlight level: Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
> > +                                             vals[0]);
> > +                     return;
> > +             }
> > +             level <<= (16 - vals[0]) & 0xFFFF;
> > +     }
> > +
> >       vals[0] = level;
> >
> >       /* Write the MSB and/or LSB */
> > diff --git a/drivers/gpu/drm/i915/display/intel_quirks.c b/drivers/gpu/drm/i915/display/intel_quirks.c
> > index 46beb155d835f..63b27d49b2864 100644
> > --- a/drivers/gpu/drm/i915/display/intel_quirks.c
> > +++ b/drivers/gpu/drm/i915/display/intel_quirks.c
> > @@ -53,6 +53,16 @@ static void quirk_increase_ddi_disabled_time(struct drm_i915_private *i915)
> >       drm_info(&i915->drm, "Applying Increase DDI Disabled quirk\n");
> >  }
> >
> > +/*
> > + * Some eDP backlight hardware uses the most-significant bits of the brightness
> > + * register, so brightness values must be shifted first.
> > + */
> > +static void quirk_shift_edp_backlight_brightness(struct drm_i915_private *i915)
> > +{
> > +     i915->quirks |= QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS;
> > +     DRM_INFO("Applying shift eDP backlight brightness quirk\n");
> > +}
> > +
> >  struct intel_quirk {
> >       int device;
> >       int subsystem_vendor;
> > @@ -156,6 +166,9 @@ static struct intel_quirk intel_quirks[] = {
> >       /* ASRock ITX*/
> >       { 0x3185, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
> >       { 0x3184, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
> > +
> > +     /* Google Pixelbook */
> > +     { 0x591E, 0x8086, 0x2212, quirk_shift_edp_backlight_brightness },
> >  };
> >
> >  void intel_init_quirks(struct drm_i915_private *i915)
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > index e4f7f6518945b..cc93bede4fab8 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -525,6 +525,7 @@ struct i915_psr {
> >  #define QUIRK_PIN_SWIZZLED_PAGES (1<<5)
> >  #define QUIRK_INCREASE_T12_DELAY (1<<6)
> >  #define QUIRK_INCREASE_DDI_DISABLED_TIME (1<<7)
> > +#define QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS (1<<8)
> >
> >  struct intel_fbdev;
> >  struct intel_fbc_work;
>
> --
> Jani Nikula, Intel Open Source Graphics Center