If the value of the clock variable is higher than 800000, the value of the
variable m, which is used as a divisor, will remain zero, because
(clock * testp) will be higher than vcomax in every loop iteration, which
leads to skipping every iteration and leaving variable m unmodified.

Clamp value of the clock variable between the lower and the upper limits.
It should be correct, since there is already a similar lower limit check.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: e829d7ef9f17 ("drm/mgag200: Add support for a new rev of G200e")
Signed-off-by: Murad Masimov <m.masimov@maxima.ru>
---
 drivers/gpu/drm/mgag200/mgag200_g200se.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/mgag200/mgag200_g200se.c b/drivers/gpu/drm/mgag200/mgag200_g200se.c
index 7a32d3b1d226..4934c27b084e 100644
--- a/drivers/gpu/drm/mgag200/mgag200_g200se.c
+++ b/drivers/gpu/drm/mgag200/mgag200_g200se.c
@@ -216,8 +216,7 @@ static int mgag200_g200se_04_pixpllc_atomic_check(struct drm_crtc *crtc,
 	m = n = p = s = 0;
 	delta = 0xffffffff;

-	if (clock < 25000)
-		clock = 25000;
+	clock = clamp(clock, 25000L, 800000L);
 	clock = clock * 2;

 	/* Permited delta is 0.5% as VESA Specification */
--
2.39.2

On 11/11/2024 17:33, Murad Masimov wrote:
> If the value of the clock variable is higher than 800000, the value of the
> variable m, which is used as a divisor, will remain zero, because
> (clock * testp) will be higher than vcomax in every loop iteration, which
> leads to skipping every iteration and leaving variable m unmodified.
> 
> Clamp value of the clock variable between the lower and the upper limits.
> It should be correct, since there is already a similar lower limit check.

I don't think it is correct.

If the clock asked is > 800000, then delta > premitteddelta, and it will 
return -EINVAL.
With your patch it will instead configure the clock to 800000 which is 
too low for the mode asked and will result in corrupted output.

Best regards,

-- 

Jocelyn

> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: e829d7ef9f17 ("drm/mgag200: Add support for a new rev of G200e")
> Signed-off-by: Murad Masimov <m.masimov@maxima.ru>
> ---
>   drivers/gpu/drm/mgag200/mgag200_g200se.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mgag200/mgag200_g200se.c b/drivers/gpu/drm/mgag200/mgag200_g200se.c
> index 7a32d3b1d226..4934c27b084e 100644
> --- a/drivers/gpu/drm/mgag200/mgag200_g200se.c
> +++ b/drivers/gpu/drm/mgag200/mgag200_g200se.c
> @@ -216,8 +216,7 @@ static int mgag200_g200se_04_pixpllc_atomic_check(struct drm_crtc *crtc,
>   	m = n = p = s = 0;
>   	delta = 0xffffffff;
> 
> -	if (clock < 25000)
> -		clock = 25000;
> +	clock = clamp(clock, 25000L, 800000L);
>   	clock = clock * 2;
> 
>   	/* Permited delta is 0.5% as VESA Specification */
> --
> 2.39.2
>

On Thu, 14. Nov 17:47, Jocelyn Falempe wrote:
> On 11/11/2024 17:33, Murad Masimov wrote:
> > If the value of the clock variable is higher than 800000, the value of the
> > variable m, which is used as a divisor, will remain zero, because
> > (clock * testp) will be higher than vcomax in every loop iteration, which
> > leads to skipping every iteration and leaving variable m unmodified.
> > 
> > Clamp value of the clock variable between the lower and the upper limits.
> > It should be correct, since there is already a similar lower limit check.
> 
> I don't think it is correct.
> 
> If the clock asked is > 800000, then delta > premitteddelta, and it will
> return -EINVAL.

In many cases when clock is > 800000, the check won't be reached as the
division by "m" variable containing a zero value will have occured just
before.

> With your patch it will instead configure the clock to 800000 which is too
> low for the mode asked and will result in corrupted output.

Worth moving the check just after the loop or e.g. explicitly denying
clocks > 800000 at the beginning of the function?