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

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Date:   Thu, 27 Aug 2020 13:18:33 +0200
From:   Christian König <christian.koenig@....com>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>,
        alexander.deucher@....com
Cc:     David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        "open list:RADEON and AMDGPU DRM DRIVERS" 
        <amd-gfx@...ts.freedesktop.org>, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH] drm/radeon: Prefer lower feedback dividers

Am 25.08.20 um 19:33 schrieb Kai-Heng Feng:
> 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.

Mhm, let's hope that this works out for all others as well :)

>
> Users confirmed this change fixes the regression and doesn't regress the
> original fix.
>
> Fixes: 2e26ccb119bd ("drm/radeon: prefer lower reference dividers")
> BugLink: https://bugs.launchpad.net/bugs/1791312
> BugLink: https://bugs.launchpad.net/bugs/1861554
> Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>

Reviewed-by: Christian König <christian.koenig@....com>

> ---
>   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 e0ae911ef427..7b69d6dfe44a 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) {