[PATCH] drm/mgag200: Apply upper limit for clock variable

[PATCH] drm/mgag200: Apply upper limit for clock variable

[Murad Masimov]

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 to the upper limit, the same way it's done
with the lower limit.

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

Fixes: 877507bb954e ("drm/mgag200: Provide per-device callbacks for PIXPLLC")
Signed-off-by: Murad Masimov <m.masimov@maxima.ru>
---
 drivers/gpu/drm/mgag200/mgag200_g200se.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/mgag200/mgag200_g200se.c b/drivers/gpu/drm/mgag200/mgag200_g200se.c
index 7a32d3b1d226..69402a591f59 100644
--- a/drivers/gpu/drm/mgag200/mgag200_g200se.c
+++ b/drivers/gpu/drm/mgag200/mgag200_g200se.c
@@ -218,6 +218,8 @@ static int mgag200_g200se_04_pixpllc_atomic_check(struct drm_crtc *crtc,

 	if (clock < 25000)
 		clock = 25000;
+	else if (clock > 800000)
+		clock = 800000;
 	clock = clock * 2;

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

Re: [PATCH] drm/mgag200: Apply upper limit for clock variable

[Christophe JAILLET]

Le 11/11/2024 à 14:46, Murad Masimov a écrit :
> 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 to the upper limit, the same way it's done
> with the lower limit.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: 877507bb954e ("drm/mgag200: Provide per-device callbacks for PIXPLLC")
> Signed-off-by: Murad Masimov <m.masimov@maxima.ru>
> ---
>   drivers/gpu/drm/mgag200/mgag200_g200se.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mgag200/mgag200_g200se.c b/drivers/gpu/drm/mgag200/mgag200_g200se.c
> index 7a32d3b1d226..69402a591f59 100644
> --- a/drivers/gpu/drm/mgag200/mgag200_g200se.c
> +++ b/drivers/gpu/drm/mgag200/mgag200_g200se.c
> @@ -218,6 +218,8 @@ static int mgag200_g200se_04_pixpllc_atomic_check(struct drm_crtc *crtc,
> 
>   	if (clock < 25000)
>   		clock = 25000;
> +	else if (clock > 800000)
> +		clock = 800000;

Maybe:
	clock = clamp(clock, 25000, 800000);

to be less verbose?

CJ

>   	clock = clock * 2;
> 
>   	/* Permited delta is 0.5% as VESA Specification */
> --
> 2.39.2
> 
> 
>