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

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Date:   Mon, 24 Feb 2020 11:44:52 +0100
From:   Torsten Duwe <duwe@....de>
To:     Icenowy Zheng <icenowy@...c.io>,
        Thomas Zimmermann <tzimmermann@...e.de>
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>,
        Sam Ravnborg <sam@...nborg.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        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 10:43:02AM +0800, Icenowy Zheng wrote:
> 
> 
> 于 2020年2月22日 GMT+08:00 上午1:13:28, Torsten Duwe <duwe@....de> 写到:
> >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?
> 
> Because the dpcd array is then written as a continous array
> back to DPCD.

But the current code never changes this value?
Anyway, as this might change in the future, I support your version;
I want to see this fixed.

Reviewed-by: Torsten Duwe <duwe@...e.de>
Fixes: e1cff82c1097 ("drm/bridge: fix anx6345 compilation for v5.5")
Cc: Maxime Ripard <maxime@...no.tech>
Cc: Torsten Duwe <duwe@....de>
Cc: Sam Ravnborg <sam@...nborg.org>
Cc: Linus Walleij <linus.walleij@...aro.org>
Cc: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Icenowy Zheng <icenowy@...c.io>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>

> >
> >>  	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
> 
> -- 
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