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

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Message-ID: <20241126-observant-coyote-of-glee-3b94dd@houat>
Date: Tue, 26 Nov 2024 13:09:10 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Sean Nyekjaer <sean@...nix.com>
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

On Tue, Nov 26, 2024 at 12:34:26PM +0100, Sean Nyekjaer wrote:
> Hi,
> 
> On Tue, Nov 26, 2024 at 09:38:55AM +0100, Maxime Ripard wrote:
> > Hi,
> > 
> > On Tue, Nov 26, 2024 at 08:36:00AM +0100, Sean Nyekjaer wrote:
> > > On Mon, Nov 25, 2024 at 05:00:56PM +0100, Maxime Ripard wrote:
> > > > 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%
> > 
> > This code was only used for panels so far. It reuses the same tolerance
> > than HDMI because we couldn't come up with anything better, but it
> > should totally apply to other things.
> > 
> > > and in my case the LVDS panel 10%.
> > 
> > 10% is a lot, and I'm not sure we'll want that. The framerate being
> > anywhere between 54 and 66 fps will trip a lot of applications too.
> > 
> > Why do you need such a big tolerance?
> 
> I don't need it, it was just from the datasheet for the LVDS panel :)

So you mean the panel accepts a pixel clock within +/- 10%?

That makes sense, but then we should also adjust the mode timings to
match so we still keep 60fps. There's much more to *that* than the
helpers you try to create though, so let's keep it aside for now.

Maxime

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