linux-kernel - Re: [PATCH 5.8 101/186] drm/radeon: Prefer lower feedback dividers

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Date:   Wed, 9 Sep 2020 13:15:13 +0200
From:   Christian König <christian.koenig@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Alex Deucher <alexander.deucher@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.8 101/186] drm/radeon: Prefer lower feedback dividers

Hi Greg,

please drop that patch. It turned out to break a lot of different setups 
and we are going to revert it now.

Thanks,
Christian.

Am 08.09.20 um 17:24 schrieb Greg Kroah-Hartman:
> From: Kai-Heng Feng <kai.heng.feng@...onical.com>
>
> [ Upstream commit fc8c70526bd30733ea8667adb8b8ffebea30a8ed ]
>
> Commit 2e26ccb119bd ("drm/radeon: prefer lower reference dividers")
> fixed screen flicker for HP Compaq nx9420 but breaks other laptops like
> Asus X50SL.
>
> Turns out we also need to favor lower feedback dividers.
>
> Users confirmed this change fixes the regression and doesn't regress the
> original fix.
>
> Fixes: 2e26ccb119bd ("drm/radeon: prefer lower reference dividers")
> BugLink: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.launchpad.net%2Fbugs%2F1791312&amp;data=02%7C01%7Cchristian.koenig%40amd.com%7C9fcd8f691f97451da30608d8540e3896%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637351767352887370&amp;sdata=KKx0nGwITXVSg5Bg7XJHbVh0T30knZDioivKxO4%2F%2BB0%3D&amp;reserved=0
> BugLink: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.launchpad.net%2Fbugs%2F1861554&amp;data=02%7C01%7Cchristian.koenig%40amd.com%7C9fcd8f691f97451da30608d8540e3896%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637351767352897339&amp;sdata=KWknTc75Zjw3clREsEfY6wtDcv%2F3gddeyHhrhlHN0nY%3D&amp;reserved=0
> Reviewed-by: Christian König <christian.koenig@....com>
> Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> Signed-off-by: Alex Deucher <alexander.deucher@....com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   drivers/gpu/drm/radeon/radeon_display.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_display.c b/drivers/gpu/drm/radeon/radeon_display.c
> index df1a7eb736517..840c4bf6307fd 100644
> --- a/drivers/gpu/drm/radeon/radeon_display.c
> +++ b/drivers/gpu/drm/radeon/radeon_display.c
> @@ -933,7 +933,7 @@ static void avivo_get_fb_ref_div(unsigned nom, unsigned den, unsigned post_div,
>   
>   	/* get matching reference and feedback divider */
>   	*ref_div = min(max(den/post_div, 1u), ref_div_max);
> -	*fb_div = DIV_ROUND_CLOSEST(nom * *ref_div * post_div, den);
> +	*fb_div = max(nom * *ref_div * post_div / den, 1u);
>   
>   	/* limit fb divider to its maximum */
>   	if (*fb_div > fb_div_max) {