Re: [PATCH v4 2/5] drm/dp: fix link probing for devices supporting DP 1.4+

[Quentin Schulz]

Hi Manasi,

On Fri, Sep 21, 2018 at 12:59:30AM -0700, Manasi Navare wrote:
> Thanks for the patch. Verified with the DP 1.4 spec and looks good to me.
> Also look at the related patch that makes use of the correct extended capabilities:
> 
> https://patchwork.freedesktop.org/patch/249400/
> 

If I'm not mistaken, you could get rid of the patch in the Intel driver
when we have this one merged. Is there any reason not to, have I missed
something?

Thanks,
Quentin

> Reviewed-by: Manasi Navare <manasi.d.navare@xxxxxxxxx>
> 
> Manasi
> 
> 
> On Thu, Sep 20, 2018 at 03:54:37PM +0100, Damian Kos wrote:
> > From: Quentin Schulz <quentin.schulz@xxxxxxxxxxxxxxxxxx>
> > 
> > DP 1.4 introduced a DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit in
> > DP_TRAINING_AUX_RD_INTERVAL register. If set, DPCD registers from
> > DP_DPCD_REV to DP_ADAPTER_CAP should be retrieved starting from
> > DP_DPCD_REV_EXTENDED. All registers are copied except DP_DPCD_REV,
> > DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT which represent the
> > "true capabilities" of DPRX device.
> > 
> > Original DP_DPCD_REV, DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT
> > might falsely return lower capabilities to "avoid interoperability
> > issues with some of the existing DP Source devices that malfunction
> > when they discover the higher capabilities within those three
> > registers.".
> > 
> > Before DP 1.4, DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit was reserved
> > and read 0 so it's safe to check against it even if DP revision is
> > <1.4
> > 
> > Signed-off-by: Quentin Schulz <quentin.schulz@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Damian Kos <dkos@xxxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/drm_dp_helper.c | 30 +++++++++++++++++++++++++++++-
> >  1 file changed, 29 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> > index 8c6b9fd89f8a..735ebde5c2f0 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -370,10 +370,38 @@ int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link)
> >  {
> >  	u8 values[3];
> >  	int err;
> > +	unsigned int addr;
> >  
> >  	memset(link, 0, sizeof(*link));
> >  
> > -	err = drm_dp_dpcd_read(aux, DP_DPCD_REV, values, sizeof(values));
> > +	/*
> > +	 * DP 1.4 introduced a DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit in
> > +	 * DP_TRAINING_AUX_RD_INTERVAL register. If set, DPCD registers from
> > +	 * DP_DPCD_REV to DP_ADAPTER_CAP should be retrieved starting from
> > +	 * DP_DPCD_REV_EXTENDED. All registers are copied except DP_DPCD_REV,
> > +	 * DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT which represent the
> > +	 * "true capabilities" of DPRX device.
> > +	 *
> > +	 * Original DP_DPCD_REV, DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT
> > +	 * might falsely return lower capabilities to "avoid interoperability
> > +	 * issues with some of the existing DP Source devices that malfunction
> > +	 * when they discover the higher capabilities within those three
> > +	 * registers.".
> > +	 *
> > +	 * Before DP 1.4, DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit was reserved
> > +	 * and read 0 so it's safe to check against it even if DP revision is
> > +	 * <1.4
> > +	 */
> > +	err = drm_dp_dpcd_readb(aux, DP_TRAINING_AUX_RD_INTERVAL, values);
> > +	if (err < 0)
> > +		return err;
> > +
> > +	if (values[0] & DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT)
> > +		addr = DP_DP13_DPCD_REV;
> > +	else
> > +		addr = DP_DPCD_REV;
> > +
> > +	err = drm_dp_dpcd_read(aux, addr, values, sizeof(values));
> >  	if (err < 0)
> >  		return err;
> >  
> > -- 
> > 2.17.1
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel

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