[PATCH] drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ

[Ville Syrjälä]

On Tue, Dec 31, 2019 at 03:30:47AM +0000, Lin, Wayne wrote:
> [AMD Official Use Only - Internal Distribution Only]
> 
> > ________________________________________
> > From: Jani Nikula <jani.nikula at linux.intel.com>
> > Sent: Monday, December 30, 2019 19:15
> > To: Lin, Wayne; dri-devel at lists.freedesktop.org; amd-gfx at lists.freedesktop.org
> > Cc: Zuo, Jerry; Kazlauskas, Nicholas; Lin, Wayne
> > Subject: Re: [PATCH] drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ
> >
> > On Mon, 30 Dec 2019, Wayne Lin <Wayne.Lin at amd.com> wrote:
> > > [Why]
> > > According to DP spec, it should shift left 4 digits for NO_STOP_BIT
> > > in REMOTE_I2C_READ message. Not 5 digits.
> > >
> > > [How]
> > > Correct the shifting value of NO_STOP_BIT for DP_REMOTE_I2C_READ case in
> > > drm_dp_encode_sideband_req().
> >
> > Which commit introduced the issue? Fixes: tag. Does it need a stable
> > backport? Does this fix a user visible bug?
> >
> > BR,
> > Jani.
> >
> Thanks for your time and reminder.
> 
> It seems like the issue has been there since very beginning.(commit: ad7f8a1).
> It doesn't introduce user visible bug under my test cases, but this affects the I2C signal
> between I2C master and I2C slave. Not pretty sure if there is any eeprom will reset
> the written offset once received I2C stop. If so, that might cause wrongly reading EDID.
> I will Cc to stable. Thanks.

The segment address should be reset on STOP. So large EDIDs should
fail. IIRC we had a bug report of some sort about this which I tried
to fix by confgiuring .no_stop_bit correctly, but apparently I failed
to double check that the bit get stuffed onto the wire correctly.

Ah yes, https://bugs.freedesktop.org/show_bug.cgi?id=108081
So you may have just fixed that one, although we seem to have closed
it already.

> > > Signed-off-by: Wayne Lin <Wayne.Lin at amd.com>
> > > ---
> > >  drivers/gpu/drm/drm_dp_mst_topology.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > index 1d1bfa49ca2b..0557e225ff67 100644
> > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > @@ -393,7 +393,7 @@ drm_dp_encode_sideband_req(const struct drm_dp_sideband_msg_req_body *req,
> > >                       memcpy(&buf[idx], req->u.i2c_read.transactions[i].bytes, req->u.i2c_read.transactions[i].num_bytes);
> > >                       idx += req->u.i2c_read.transactions[i].num_bytes;
> > >
> > > -                     buf[idx] = (req->u.i2c_read.transactions[i].no_stop_bit & 0x1) << 5;
> > > +                     buf[idx] = (req->u.i2c_read.transactions[i].no_stop_bit & 0x1) << 4;
> > >                       buf[idx] |= (req->u.i2c_read.transactions[i].i2c_transaction_delay & 0xf);
> > >                       idx++;
> > >               }
> >
> > --
> > Jani Nikula, Intel Open Source Graphics Center
> --
> Wayne Lin
> _______________________________________________
> dri-devel mailing list
> dri-devel at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Ville Syrjälä
Intel