linux-kernel - Re: [PATCH v2 1/3] drm/modes: introduce drm_mode_validate

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Message-ID: <bfuj6w6hsbfpdw24th6dl3ugvj45op6jb45gx5ab5pulud7hiz@o2zbn45z3lt4>
Date: Tue, 26 Nov 2024 08:36:00 +0100
From: Sean Nyekjaer <sean@...nix.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Chen-Yu Tsai <wens@...e.org>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, 
	Yannick Fertre <yannick.fertre@...s.st.com>, Raphael Gallais-Pou <raphael.gallais-pou@...s.st.com>, 
	Philippe Cornu <philippe.cornu@...s.st.com>, Maxime Coquelin <mcoquelin.stm32@...il.com>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev, 
	linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH v2 1/3] drm/modes: introduce drm_mode_validate_mode()
 helper function

Hi Maxime,

On Mon, Nov 25, 2024 at 05:00:56PM +0100, Maxime Ripard wrote:
> Hi Sean,
> 
> On Mon, Nov 25, 2024 at 02:49:26PM +0100, Sean Nyekjaer wrote:
> > Check if the required pixel clock is in within .5% range of the
> > desired pixel clock.
> > This will match the requirement for HDMI where a .5% tolerance is allowed.
> > 
> > Signed-off-by: Sean Nyekjaer <sean@...nix.com>
> > ---
> >  drivers/gpu/drm/drm_modes.c | 34 ++++++++++++++++++++++++++++++++++
> >  include/drm/drm_modes.h     |  2 ++
> >  2 files changed, 36 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> > index 6ba167a3346134072d100af0adbbe9b49e970769..4068b904759bf80502efde6e4d977b297f5d5359 100644
> > --- a/drivers/gpu/drm/drm_modes.c
> > +++ b/drivers/gpu/drm/drm_modes.c
> > @@ -1623,6 +1623,40 @@ bool drm_mode_equal_no_clocks_no_stereo(const struct drm_display_mode *mode1,
> >  }
> >  EXPORT_SYMBOL(drm_mode_equal_no_clocks_no_stereo);
> >  
> > +/**
> > + * drm_mode_validate_mode
> > + * @mode: mode to check
> > + * @rounded_rate: output pixel clock
> > + *
> > + * VESA DMT defines a tolerance of 0.5% on the pixel clock, while the
> > + * CVT spec reuses that tolerance in its examples, so it looks to be a
> > + * good default tolerance for the EDID-based modes. Define it to 5 per
> > + * mille to avoid floating point operations.
> > + *
> > + * Returns:
> > + * The mode status
> > + */
> > +enum drm_mode_status drm_mode_validate_mode(const struct drm_display_mode *mode,
> > +					    unsigned long long rounded_rate)
> > +{
> > +	enum drm_mode_status status;
> > +	unsigned long long rate = mode->clock * 1000;
> > +	unsigned long long lowest, highest;
> > +
> > +	lowest = rate * (1000 - 5);
> > +	do_div(lowest, 1000);
> > +	if (rounded_rate < lowest)
> > +		return MODE_CLOCK_LOW;
> > +
> > +	highest = rate * (1000 + 5);
> > +	do_div(highest, 1000);
> > +	if (rounded_rate > highest)
> > +		return MODE_CLOCK_HIGH;
> > +
> > +	return MODE_OK;
> > +}
> > +EXPORT_SYMBOL(drm_mode_validate_mode);
> 
> Thanks a lot for doing that!
> 
> I wonder about the naming though (and prototype). I doesn't really
> validates a mode, but rather makes sure that a given rate is a good
> approximation of a pixel clock. So maybe something like
> drm_mode_check_pixel_clock?

Naming is hard :) I will use drm_mode_check_pixel_clock() for V2.

Would it make sense to have the pixel clock requirement as a input
parameter? For HDMI it is 0.5% and in my case the LVDS panel 10%.

enum drm_mode_status drm_mode_validate_mode(const struct drm_display_mode *mode,
					    unsigned long long rounded_rate, unsigned tolerance)
?

And maybe a drm_mode_validate_mode_hdmi() with the default tolerance of
.5%?
> 
> We probably need some kunit tests here too.

Good idea, will be my first :)

/Sean