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

[Chris Wilson]

Quoting Ville Syrjälä (2020-10-30 14:43:46)
> 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 at 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 at vger.kernel.org
> > > Reported-by: Randy Dunlap <rdunlap at infradead.org>
> > > Signed-off-by: Ville Syrjälä <ville.syrjala at 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.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris