linux-kernel - Re: [PATCH] drm/bridge: analogix-anx6345: fix set of link bandwidth

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Message-ID: <20200221171328.GC6928@lst.de>
Date:   Fri, 21 Feb 2020 18:13:28 +0100
From:   Torsten Duwe <duwe@....de>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Maxime Ripard <maxime@...no.tech>,
        Vasily Khoruzhick <anarsoul@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/bridge: analogix-anx6345: fix set of link bandwidth

On Sat, Feb 22, 2020 at 12:51:27AM +0800, Icenowy Zheng wrote:
> Current code tries to store the link rate (in bps, which is a big
> number) in a u8, which surely overflow. Then it's converted back to
> bandwidth code (which is thus 0) and written to the chip.
> 
> The code sometimes works because the chip will automatically fallback to
> the lowest possible DP link rate (1.62Gbps) when get the invalid value.
> However, on the eDP panel of Olimex TERES-I, which wants 2.7Gbps link,
> it failed.
> 
> As we had already read the link bandwidth as bandwidth code in earlier
> code (to check whether it is supported), use it when setting bandwidth,
> instead of converting it to link rate and then converting back.
> 
> Fixes: e1cff82c1097 ("drm/bridge: fix anx6345 compilation for v5.5")
> Signed-off-by: Icenowy Zheng <icenowy@...c.io>
> ---
>  drivers/gpu/drm/bridge/analogix/analogix-anx6345.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/analogix/analogix-anx6345.c b/drivers/gpu/drm/bridge/analogix/analogix-anx6345.c
> index 56f55c53abfd..2dfa2fd2a23b 100644
> --- a/drivers/gpu/drm/bridge/analogix/analogix-anx6345.c
> +++ b/drivers/gpu/drm/bridge/analogix/analogix-anx6345.c
> @@ -210,8 +210,7 @@ static int anx6345_dp_link_training(struct anx6345 *anx6345)
>  	if (err)
>  		return err;
>  
> -	dpcd[0] = drm_dp_max_link_rate(anx6345->dpcd);
> -	dpcd[0] = drm_dp_link_rate_to_bw_code(dpcd[0]);
> +	dpcd[0] = dp_bw;

Why do you make this assignment and not use dp_bw directly in the call?

>  	err = regmap_write(anx6345->map[I2C_IDX_DPTX],
>  			   SP_DP_MAIN_LINK_BW_SET_REG, dpcd[0]);
                                                       ^^^^^^
>  	if (err)
> -- 
> 2.24.1

BTW, my version is only a bit more verbose:

https://patchwork.freedesktop.org/patch/354344/

	Torsten