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

[Ville Syrjala]

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;
 
 	if (mode->flags & DRM_MODE_FLAG_INTERLACE)
@@ -772,7 +772,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
 	if (mode->vscan > 1)
 		den *= mode->vscan;
 
-	return DIV_ROUND_CLOSEST(num, den);
+	return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(num, 1000), den);
 }
 EXPORT_SYMBOL(drm_mode_vrefresh);
 
-- 
2.26.2