Message-ID: <20200917201102.GD6112@intel.com>
Date:   Thu, 17 Sep 2020 23:11:02 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Puthikorn Voravootivat <puthik@...omium.org>
Cc:     Kevin Chowski <chowski@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org,
        Wambui Karuga <wambui.karugax@...il.com>
Subject: Re: [Intel-gfx] [PATCH] i915: Introduce quirk for shifting eDP
 brightness.

On Thu, Sep 17, 2020 at 09:25:35PM +0300, Ville Syrjälä wrote:
> On Thu, Sep 17, 2020 at 11:14:43AM -0700, Puthikorn Voravootivat wrote:
> > The Lyude fde7266fb2f6 change is actually based on Chromium change
> > (https://crrev.com/c/1650325) that fixes the brightness for Samsung
> > Galaxy Chromebook. So currently we have 2 examples that use LSB
> > interpretation and 1 that use MSB.
> 
> "If field 4:0 of the EDP_PWMGEN_BIT_COUNT register represents a value
> of greater than 8 and the BACKLIGHT_BRIGHTNESS_BYTE_COUNT bit
> is cleared to 0, only the 8 MSB of the brightness control value can be
> controlled.
> (See Note below.)
> Assigned bits are allocated to the MSB of the enabled register
> combination."
> 
> I think that's pretty clear the assigned bits are supposed to be
> msb aligned.

I guess there's some email issues happening, but just to clarify:

When the spec says MSB in caps here it clearly means
"most significant-bit(s)" since otherwise "8 MSB" would not make
any sense in the context of a 2 byte value.

Granted the spec is crap here since "Table 1-1: Acronyms and
Initialism" does claim "MSB" should be byte(s) and "msb" bit(s).

Also I can't imagine anyone would allocate the bits starting
from the lsb when the whole thing is clearly supposed to be a
16bit big endian integer. So with >8 assigned bits you'd end
up with crazy stuff like this:

[ 7 ... 0 ][7   ...   0]
[ 8 MSB   ][XXXX][N LSB]

so you couldn't even treat the value as a regular big endian
thing. Instead, if you squint a bit, it now looks like a funky
little endian value. So we're deep into some mixed endian land
where nothing makes sense anymore.

Anyways I think the code should simply do this to match the spec:
u16 value = brightness << (16 - num_assigned_bits);
val[0] = value >> 8;
val[1] = value & 0xff;


> 
> > 
> > 
> > On Thu, Sep 17, 2020 at 10:55 AM Kevin Chowski <chowski@...omium.org> wrote:
> > >
> > > 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@...ux.intel.com> wrote:
> > > >
> > > > On Thu, 17 Sep 2020, Kevin Chowski <chowski@...omium.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@...omium.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
> 
> -- 
> Ville Syrjälä
> Intel
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Ville Syrjälä
Intel

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