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

[Lin, Wayne]

[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.
> > 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