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

[Lin, Wayne]

[AMD Public Use]



> -----Original Message-----
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Sent: Wednesday, January 8, 2020 2:57 AM
> To: Lin, Wayne <Wayne.Lin at amd.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>; dri-
> devel at lists.freedesktop.org; amd-gfx at lists.freedesktop.org; Zuo, Jerry
> <Jerry.Zuo at amd.com>; Kazlauskas, Nicholas
> <Nicholas.Kazlauskas at amd.com>
> Subject: Re: [PATCH] drm/dp_mst: correct the shifting in
> DP_REMOTE_I2C_READ
> 
> 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.

Thanks for your time and the explanation.
> 
> > > > 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
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> --
> Ville Syrjälä
> Intel
--
Best regards,
Wayne Lin