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

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Message-ID: <20241125-gleaming-anteater-of-perfection-42bd2b@houat>
Date: Mon, 25 Nov 2024 17:00:56 +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

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?

We probably need some kunit tests here too.

Maxime

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