linux-kernel - Re: [PATCH v2] drm/mgag200: Apply upper limit for clock variable

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Message-ID: <a394af10-8428-4d19-84cf-f21ec63f23f5@redhat.com>
Date: Thu, 14 Nov 2024 17:47:46 +0100
From: Jocelyn Falempe <jfalempe@...hat.com>
To: Murad Masimov <m.masimov@...ima.ru>, Dave Airlie <airlied@...hat.com>
Cc: Thomas Zimmermann <tzimmermann@...e.de>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH v2] drm/mgag200: Apply upper limit for clock variable

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@...ima.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
>