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On 10/12/17 12:53, Kevin Hao wrote:
> On Thu, Oct 12, 2017 at 12:39:25PM +0300, Jyri Sarha wrote:
>> We are using the vrefresh to check if we are too close to vertical
>> sync to update the two framebuffer DMA registers and risk a
>> collision. The vrefresh is coming from user space and normally it is
>> not used for anything. For instance xserver leaves vrefresh to zero
>> causing a division by zero when setting the mode. We want to make sure
>> the value is valid and force its recalculation in
>> tilcdc_crtc_set_mode().
>>
>> Reported-by: Kevin Hao <kexin.hao@xxxxxxxxxxxxx>
>> Fixes: 11abbc9f39e0 ("drm/tilcdc: Set framebuffer DMA address to HW only if CRTC is enabled")
>> Cc: <stable@xxxxxxxxxxxxxxx> # v4.11+
>> Signed-off-by: Jyri Sarha <jsarha@xxxxxx>
>> ---
>> drivers/gpu/drm/tilcdc/tilcdc_crtc.c | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_crtc.c b/drivers/gpu/drm/tilcdc/tilcdc_crtc.c
>> index 704db24..aa5e87c 100644
>> --- a/drivers/gpu/drm/tilcdc/tilcdc_crtc.c
>> +++ b/drivers/gpu/drm/tilcdc/tilcdc_crtc.c
>> @@ -459,6 +459,16 @@ static void tilcdc_crtc_set_mode(struct drm_crtc *crtc)
>> drm_framebuffer_get(fb);
>>
>> crtc->hwmode = crtc->state->adjusted_mode;
>> +
>> + /* We are using the vrefresh to check if we are too close to
>> + * vertical sync to update the two framebuffer DMA registers
>> + * and risk a collision. The vrefresh is coming from user
>> + * space and normally it is not used for anything. We want to
>> + * make sure the value is valid and force its recalculation
>> + * here.
>> + */
>> + crtc->hwmode.vrefresh = 0;
>
> Why do we need this?
>
The drm_mode_vrefresh() [1] checks if the vrefresh is > 0 and only
calculates it if it is not. I want to make sure the value is correct by
recalculating it always. This is to prevent buggy user space SW from
causing undefined behaviour.
Best regards,
Jyri
[1] int drm_mode_vrefresh(const struct drm_display_mode *mode)
{
int refresh = 0;
unsigned int calc_val;
if (mode->vrefresh > 0)
refresh = mode->vrefresh;
else if (mode->htotal > 0 && mode->vtotal > 0) {
int vtotal;
vtotal = mode->vtotal;
/* work out vrefresh the value will be x1000 */
calc_val = (mode->clock * 1000);
calc_val /= mode->htotal;
refresh = (calc_val + vtotal / 2) / vtotal;
if (mode->flags & DRM_MODE_FLAG_INTERLACE)
refresh *= 2;
if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
refresh /= 2;
if (mode->vscan > 1)
refresh /= mode->vscan;
}
return refresh;
}