linux-kernel - Re: [PATCH v3 6/9] drm/bridge: ti-sn65dsi86: Use 18-bit DP if we can

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Date:   Thu, 9 Jul 2020 22:17:14 -0500
From:   Steev Klimaszewski <steev@...i.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        David Airlie <airlied@...ux.ie>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Jeffrey Hugo <jeffrey.l.hugo@...il.com>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Rob Clark <robdclark@...omium.org>,
        Rob Clark <robdclark@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Steev Klimaszewski <steev@...too.org>
Subject: Re: [PATCH v3 6/9] drm/bridge: ti-sn65dsi86: Use 18-bit DP if we can


On 7/9/20 10:12 PM, Steev Klimaszewski wrote:
>
> On 7/9/20 9:14 PM, Doug Anderson wrote:
>> Hi,
>>
>> On Thu, Jul 9, 2020 at 6:38 PM Doug Anderson <dianders@...omium.org> 
>> wrote:
>>> Hi,
>>>
>>> On Thu, Jul 9, 2020 at 6:19 PM Steev Klimaszewski <steev@...too.org> 
>>> wrote:
>>>> Hi Doug,
>>>>
>>>> I've been testing 5.8 and linux-next on the Lenovo Yoga C630, and 
>>>> with this patch applied, there is really bad banding on the display.
>>>>
>>>> I'm really bad at explaining it, but you can see the differences in 
>>>> the following:
>>>>
>>>> 24bit (pre-5.8) - https://dev.gentoo.org/~steev/files/image0.jpg
>>>>
>>>> 18bit (5.8/linux-next) - 
>>>> https://dev.gentoo.org/~steev/files/image1.jpg
>>> Presumably this means that your panel is defined improperly? If the
>>> panel reports that it's a 6 bits per pixel panel but it's actually an
>>> 8 bits per pixel panel then you'll run into this problem.
>>>
>>> I would have to assume you have a bunch of out of tree patches to
>>> support your hardware since I don't see any device trees in linuxnext
>>> (other than cheza) that use this bridge chip.  Otherwise I could try
>>> to check and confirm that was the problem.
>> Ah, interesting.  Maybe you have the panel:
>>
>> boe,nv133fhm-n61
>>
>> As far as I can tell from the datasheet (I have the similar
>> boe,nv133fhm-n62) this is a 6bpp panel.  ...but if you feed it 8bpp
>> the banding goes away!  Maybe the panel itself knows how to dither???
>> ...or maybe the datasheet / edid are wrong and this is actually an
>> 8bpp panel.  Seems unlikely...
>>
>> In any case, one fix is to pick
>> <https://lore.kernel.org/dri-devel/1593087419-903-1-git-send-email-kalyan_t@codeaurora.org/>, 
>>
>> though right now that patch is only enabled for sc7180.  Maybe you
>> could figure out how to apply it to your hardware?
>>
>> ...another fix would be to pretend that your panel is 8bpp even though
>> it's actually 6bpp.  Ironically if anyone ever tried to configure BPP
>> from the EDID they'd go back to 6bpp.  You can read the EDID of your
>> panel with this:
>>
>> bus=$(i2cdetect -l | grep sn65 | sed 's/i2c-\([0-9]*\).*$/\1/')
>> i2cdump ${bus} 0x50 i
>>
>> When I do that and then decode it on the "boe,nv133fhm-n62" panel, I 
>> find:
>>
>> 6 bits per primary color channel
>>
>> -Doug
>
>
> Hi Doug,
>
> Decoding it does show be to boe,nv133fhm-n61 - and yeah it does say 
> it's 6-bit according to panelook's specs for it.
>
>
> I'll take a look at the patch and see what I can come up with... at 
> the moment, I'm forcing it to be 8bit and that does "work fine" but 
> I'd like it to be fixed properly instead of my hack.
>
> Thanks for your time and work!
>
> -- Steev
>
For what it's worth - the 5.8 that I'm testing is at 
https://github.com/steev/linux/commits/c630-5.8-rc4-inline-encryption