[PATCH v6 1/4] drm/i915: Keep track of pwm-related backlight hooks separately

[Lyude Paul]

On Thu, 2021-01-14 at 09:12 +0200, Jani Nikula wrote:
> On Wed, 13 Jan 2021, Lyude Paul <lyude at redhat.com> wrote:
> > Currently, every different type of backlight hook that i915 supports is
> > pretty straight forward - you have a backlight, probably through PWM
> > (but maybe DPCD), with a single set of platform-specific hooks that are
> > used for controlling it.
> > 
> > HDR backlights, in particular VESA and Intel's HDR backlight
> > implementations, can end up being more complicated. With Intel's
> > proprietary interface, HDR backlight controls always run through the
> > DPCD. When the backlight is in SDR backlight mode however, the driver
> > may need to bypass the TCON and control the backlight directly through
> > PWM.
> > 
> > So, in order to support this we'll need to split our backlight callbacks
> > into two groups: a set of high-level backlight control callbacks in
> > intel_panel, and an additional set of pwm-specific backlight control
> > callbacks. This also implies a functional changes for how these
> > callbacks are used:
> > 
> > * We now keep track of two separate backlight level ranges, one for the
> >   high-level backlight, and one for the pwm backlight range
> > * We also keep track of backlight enablement and PWM backlight
> >   enablement separately
> > * Since the currently set backlight level might not be the same as the
> >   currently programmed PWM backlight level, we stop setting
> >   panel->backlight.level with the currently programmed PWM backlight
> >   level in panel->backlight.pwm_funcs->setup(). Instead, we rely
> >   on the higher level backlight control functions to retrieve the
> >   current PWM backlight level (in this case, intel_pwm_get_backlight()).
> >   Note that there are still a few PWM backlight setup callbacks that
> >   do actually need to retrieve the current PWM backlight level, although
> >   we no longer save this value in panel->backlight.level like before.
> > 
> > Additionally, we drop the call to lpt_get_backlight() in
> > lpt_setup_backlight(), and avoid unconditionally writing the PWM value that
> > we get from it and only write it back if we're in CPU mode, and switching
> > to PCH mode. The reason for this is because in the original codepath for
> > this, it was expected that the intel_panel_bl_funcs->setup() hook would be
> > responsible for fetching the initial backlight level. On lpt systems, the
> > only time we could ever be in PCH backlight mode is during the initial
> > driver load - meaning that outside of the setup() hook, lpt_get_backlight()
> > will always be the callback used for retrieving the current backlight
> > level. After this patch we still need to fetch and write-back the PCH
> > backlight value if we're switching from CPU mode to PCH, but because
> > intel_pwm_setup_backlight() will retrieve the backlight level after setup()
> > using the get() hook, which always ends up being lpt_get_backlight(). Thus
> > - an additional call to lpt_get_backlight() in lpt_setup_backlight() is
> > made redundant.
> > 
> > v7:
> > * Use panel->backlight.pwm_funcs->get() to get the backlight level in
> >   intel_pwm_setup_backlight(), lest we upset lockdep
> 
> I think this change is wrong, as it now bypasses
> intel_panel_invert_pwm_level(). Please explain. I don't see anything in
> there that could trigger a lockdep warning.

yeah-this was definitely me misunderstanding what the issue we were hitting here
was.

> 
> Perhaps it's the below you're referring to, but I think the root cause
> is different?
> 
> > @@ -1788,22 +1780,17 @@ static int vlv_setup_backlight(struct
> > intel_connector *connector, enum pipe pipe
> >         panel->backlight.active_low_pwm = ctl2 & BLM_POLARITY_I965;
> >  
> >         ctl = intel_de_read(dev_priv, VLV_BLC_PWM_CTL(pipe));
> > -       panel->backlight.max = ctl >> 16;
> > +       panel->backlight.pwm_level_max = ctl >> 16;
> >  
> > -       if (!panel->backlight.max)
> > -               panel->backlight.max = get_backlight_max_vbt(connector);
> > +       if (!panel->backlight.pwm_level_max)
> > +               panel->backlight.pwm_level_max =
> > get_backlight_max_vbt(connector);
> >  
> > -       if (!panel->backlight.max)
> > +       if (!panel->backlight.pwm_level_max)
> >                 return -ENODEV;
> >  
> > -       panel->backlight.min = get_backlight_min_vbt(connector);
> > +       panel->backlight.pwm_level_min = get_backlight_min_vbt(connector);
> >  
> > -       val = _vlv_get_backlight(dev_priv, pipe);
> 
> Turns out this is a meaningful change, as the higher level
> vlv_get_backlight() function that will be called instead hits:
> 
> <4>[   12.870202] i915 0000:00:02.0: drm_WARN_ON(!drm_modeset_is_locked(&dev-
> >mode_config.connection_mutex))
> 
> in intel_connector_get_pipe(connector).
> 
> It's a real problem. See this, it's obvious (in retrospect):
> 
>  
> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_19348/fi-bsw-kefka/igt@runner@aborted.html
>  
> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_19348/fi-bsw-kefka/boot0.txt
> 
> I don't have a quick answer how this could be handled neatly. Perhaps
> the ->get call (or rather, intel_pwm_get_backlight) to set
> panel->backlight.level needs to be spread out to the end of each
> pwm_funcs->setup function after all? Though it's at the wrong
> abstraction level wrt level being a higher level, uh, level.
> 
> I don't think it's enough to just grab connection_mutex around setup
> (and even checking if we can do that is a bunch of digging) - I think
> it's likely intel_connector_get_pipe() returns INVALID_PIPE at that
> point.
> 
> Okay, here's a clumsy suggestion that I think works around this and
> unblocks the series until we figure out a better way:
> 
> 1. At the end of vlv_setup_backlight():
> 
>         /* add fixme comment about how wrong this is */
>         panel->backlight.level = intel_panel_invert_pwm_level(connector,
> _vlv_get_backlight());
>         
> 
> 2. In intel_pwm_setup_backlight() only set level if ->setup didn't:
> 
>         if (!panel->backlight.level)
>                 panel->backlight.level = intel_pwm_get_backlight(connector);
> 
> What do you think?

Hm, I might have a better idea. Why not just adjust struct intel_panel_bl_funcs
so that it accepts an enum pipe, since we're already being passed a pipe in -
>setup(). Then in places where we call ->get() we can just make retrieving the
currently set pipe from the atomic state or somewhere else the responsibility of
the caller. I think I'm going to add an additional patch to give this a shot and
see how it goes.
> 
> BR,
> Jani.
> 
> > -       val = intel_panel_compute_brightness(connector, val);
> > -       panel->backlight.level = clamp(val, panel->backlight.min,
> > -                                      panel->backlight.max);
> > -
> > -       panel->backlight.enabled = ctl2 & BLM_PWM_ENABLE;
> > +       panel->backlight.pwm_enabled = ctl2 & BLM_PWM_ENABLE;
> >  
> >         return 0;
> >  }
> > @@ -1828,24 +1815,18 @@ bxt_setup_backlight(struct intel_connector
> > *connector, enum pipe unused)
> 

-- 
Sincerely,
   Lyude Paul (she/her)
   Software Engineer at Red Hat
   
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