linux-kernel - Re: [PATCH v2] drm/mediatek: mtk_dsi: Avoid EPROBE

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Date:   Tue, 11 Jan 2022 12:05:56 +0100
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     CK Hu <ck.hu@...iatek.com>, dri-devel@...ts.freedesktop.org
Cc:     linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        chunkuang.hu@...nel.org, p.zabel@...gutronix.de, airlied@...ux.ie,
        daniel@...ll.ch, matthias.bgg@...il.com, kernel@...labora.com,
        andrzej.hajda@...el.com
Subject: Re: [PATCH v2] drm/mediatek: mtk_dsi: Avoid EPROBE_DEFER loop with
 external bridge

Il 06/01/22 06:22, CK Hu ha scritto:
> Hi, Angelo:
> 
> On Tue, 2022-01-04 at 10:59 +0100, AngeloGioacchino Del Regno wrote:
>> DRM bridge drivers are now attaching their DSI device at probe time,
>> which requires us to register our DSI host in order to let the bridge
>> to probe: this recently started producing an endless -EPROBE_DEFER
>> loop on some machines that are using external bridges, like the
>> parade-ps8640, found on the ACER Chromebook R13.
>>
>> Now that the DSI hosts/devices probe sequence is documented, we can
>> do adjustments to the mtk_dsi driver as to both fix now and make sure
>> to avoid this situation in the future: for this, following what is
>> documented in drm_bridge.c, move the mtk_dsi component_add() to the
>> mtk_dsi_ops.attach callback and delete it in the detach callback;
>> keeping in mind that we are registering a drm_bridge for our DSI,
>> which is only used/attached if the DSI Host is bound, it wouldn't
>> make sense to keep adding our bridge at probe time (as it would
>> be useless to have it if mtk_dsi_ops.attach() fails!), so also move
>> that one to the dsi host attach function (and remove it in detach).
> 
> This patch looks good to me, but please add 'Fixes' tag to note to
> which patch this patch want to fix.
> 
> Regards,
> CK
> 

Hello,
unfortunately I couldn't find a valid commit for a Fixes tag. This
started being an issue at some point, when the DRM API was changed to
adhere to the documented probe sequence: the MediaTek DSI driver was
not the only one that got broken/affected by these changes.

If you have any advice on which commit should be tagged, I'm open for
any kind of suggestion.

Thanks,
- Angelo