linux-kernel - Re: [PATCH v3] drm/dp

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Message-ID: <CAPM=9txafoQNfMFf0Ff1SnBTgq6jYyvjJyjCSJua6-SJVVkScQ@mail.gmail.com>
Date:   Fri, 31 Jan 2020 09:22:00 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Lyude Paul <lyude@...hat.com>
Cc:     Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Benjamin Gaignard <benjamin.gaignard@...com>,
        David Airlie <airlied@...ux.ie>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sean Paul <sean@...rly.run>
Subject: Re: [PATCH v3] drm/dp_mst: Fix W=1 warnings

> > > >
> > > hi-actually yes, we should probably be using this instead of just dropping
> > > this. Also, I didn't write this code originally I just refactored a bunch
> > > of
> > > it - Dave Airlied is the original author, but the original version of this
> > > code was written ages ago. tbh, I think it's a safe bet to say that they
> > > probably did mean to use this but forgot to and no one noticed until now.
> >
> > Hi,
> >
> > Any clue about how to use crc value ? Does it have to be checked
> > against something else ?
> > If crc are not matching what should we do of the data copied just before ?
>
> We should be able to just take the CRC value from the sideband message and
> then generate our own CRC value using the sideband message contents, and check
> if the two are equal. If they aren't, something went wrong and we didn't
> receive the message properly.
>
> Now as to what we should do when we have CRC mismatches? That's a bit more
> difficult. If you have access to the DP MST spec, I suppose a place to start
> figuring that out would be checking if there's a way for us to request that a
> branch device resend whatever message it sent previously. If there isn't, I
> guess we should just print an error in dmesg (possibly with a hexdump of the
> failed message as well) and not forward the message to the driver. Not sure of
> any better way of handling it then that

Yeah I think this reflects what I wanted to do, I've no memory of a
retransmit option in the spec, but I've away from it for a while. But
we'd want to compare the CRC with what we got to make sure the are the
same.

Dave.