linux-kernel - Re: [PATCH v2 08/15] drm/bridge: tc358767: Increase AUX transfer length limit

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Date:   Fri, 22 Mar 2019 12:01:07 -0700
From:   Andrey Smirnov <andrew.smirnov@...il.com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     dri-devel@...ts.freedesktop.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Andrey Gusakov <andrey.gusakov@...entembedded.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Chris Healy <cphealy@...il.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 08/15] drm/bridge: tc358767: Increase AUX transfer
 length limit

On Fri, Mar 22, 2019 at 6:14 AM Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>
> On 22/03/2019 05:28, Andrey Smirnov wrote:
> > According to the datasheet tc358767 can transfer up to 16 bytes via
> > its AUX channel, so the artificial limit of 8 apperas to be too
> > low. However only up to 15-bytes seem to be actually supported and
> > trying to use 16-byte transfers results in transfers failing
> > sporadically (with bogus status in case of I2C transfers), so limit it
> > to 15.
>
> 16 is the limit from the DP spec. I agree, 8 looks odd.
>
> 15 looks odd too, so I think it warrants a comment there in the code.
>

Crap, was going to add that, but forgot. Will do in v2.

> Does 15 byte transfers ever work? Or mostly works but sometimes fails?
>

15 bytes transfers work every time (at least to extent I tested it).
For 16 byte transfers it depends on the transfer type. AUX transfers
work for a while but then fail (as tested by dd'ing AUX chardev). I2C
transfers work intermittently and when they fail return completely
bogus status.

Thanks,
Andrey Smirnov