[PATCH] drm: Use u64 for intermediate dotclock calculations

[Alex Deucher]

On Fri, Oct 21, 2016 at 10:15 AM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> We have reached the era where monitor bandwidths now exceed 31bits in
> frequency calculations, though as we stored them in kHz units we are
> safe from overflow in the modelines for some time.
>
> [   48.723720] UBSAN: Undefined behaviour in ../drivers/gpu/drm/drm_modes.c:325:49
> [   48.726943] signed integer overflow:
> [   48.728503] 2240 * 1000000 cannot be represented in type 'int'
>
> Reported-by: Martin Liška <marxin.liska at gmail.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98372
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Reviewed-by: Alex Deucher <alexander.deucher at amd.com>

> ---
>  drivers/gpu/drm/drm_modes.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> index 173b7d335834..f64ac86deb84 100644
> --- a/drivers/gpu/drm/drm_modes.c
> +++ b/drivers/gpu/drm/drm_modes.c
> @@ -165,6 +165,7 @@ struct drm_display_mode *drm_cvt_mode(struct drm_device *dev, int hdisplay,
>         unsigned int vfieldrate, hperiod;
>         int hdisplay_rnd, hmargin, vdisplay_rnd, vmargin, vsync;
>         int interlace;
> +       u64 tmp;
>
>         /* allocate the drm_display_mode structure. If failure, we will
>          * return directly
> @@ -322,8 +323,11 @@ struct drm_display_mode *drm_cvt_mode(struct drm_device *dev, int hdisplay,
>                 drm_mode->vsync_end = drm_mode->vsync_start + vsync;
>         }
>         /* 15/13. Find pixel clock frequency (kHz for xf86) */
> -       drm_mode->clock = drm_mode->htotal * HV_FACTOR * 1000 / hperiod;
> -       drm_mode->clock -= drm_mode->clock % CVT_CLOCK_STEP;
> +       tmp = drm_mode->htotal; /* perform intermediate calcs in u64 */
> +       tmp *= HV_FACTOR * 1000;
> +       do_div(tmp, hperiod);
> +       tmp -= drm_mode->clock % CVT_CLOCK_STEP;
> +       drm_mode->clock = tmp;
>         /* 18/16. Find actual vertical frame frequency */
>         /* ignore - just set the mode flag for interlaced */
>         if (interlaced) {
> --
> 2.9.3
>
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