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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241126-silver-skylark-of-expertise-bba576@houat>
Date: Tue, 26 Nov 2024 10:30:07 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Raphael Gallais-Pou <raphael.gallais-pou@...s.st.com>
Cc: Sean Nyekjaer <sean@...nix.com>, 
	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>, 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 09:47:17AM +0100, Raphael Gallais-Pou wrote:
> 
> On 11/26/24 08:36, Sean Nyekjaer wrote:
> > 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);
> Hi Sean, Maxime,
> >> 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)
> > ?
> 
> 
> IMO adding the tolerance as input parameter is a good idea.  This
> would useful other than for HDMI pixel clock validation (and LVDS in
> your case).

It depends on the intent. If it's justified, why not. If it's to
workaround another issue, absolutely not. And so, generally speaking, I
don't think it's a good idea.

Maxime

Download attachment "signature.asc" of type "application/pgp-signature" (274 bytes)