Re: [Intel-gfx] [PATCH] drm/modes: Switch to 64bit maths to avoid integer overflow

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Oct 30, 2020 at 02:19:45PM +0000, Chris Wilson wrote:
> Quoting Ville Syrjala (2020-10-22 20:42:56)
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > The new >8k CEA modes have dotclocks reaching 5.94 GHz, which
> > means our clock*1000 will now overflow the 32bit unsigned
> > integer. Switch to 64bit maths to avoid it.
> > 
> > Cc: stable@vger.kernel.org
> > Reported-by: Randy Dunlap <rdunlap@infradead.org>
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> > An interesting question how many other place might suffer from similar
> > overflows. I think i915 should be mostly OK. The one place I know we use
> > Hz instead kHz is the hsw DPLL code, which I would prefer we also change
> > to use kHz. The other concern is whether we have any potential overflows
> > before we check this against the platform's max dotclock.
> > 
> > I do have this unreviewed igt series 
> > https://patchwork.freedesktop.org/series/69531/ which extends the
> > current testing with some other forms of invalid modes. Could probably
> > extend that with a mode.clock=INT_MAX test to see if anything else might
> > trip up.
> > 
> > No idea about other drivers.
> > 
> >  drivers/gpu/drm/drm_modes.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> > index 501b4fe55a3d..511cde5c7fa6 100644
> > --- a/drivers/gpu/drm/drm_modes.c
> > +++ b/drivers/gpu/drm/drm_modes.c
> > @@ -762,7 +762,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
> >         if (mode->htotal == 0 || mode->vtotal == 0)
> >                 return 0;
> >  
> > -       num = mode->clock * 1000;
> > +       num = mode->clock;
> >         den = mode->htotal * mode->vtotal;
> 
> You don't want to promote den to u64 while you are here? We are at
> 8kx4k, throw in dblscan and some vscan, and we could soon have wacky
> refresh rates.

i915 has 16kx8k hard limit currently, and we reject vscan>1
(wish we could also reject DBLSCAN). So we should not hit
that, at least not yet. Other drivers might not be so strict
I guess.

I have a nagging feeling that other places are in danger of
overflows if we try to push the current limits significantly.
But I guess no real harm in going full 64bit here, except
maybe making it a bit slower.

-- 
Ville Syrjälä
Intel
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