Re: [Intel-gfx] [PATCH] drm/dp: Correctly mask DP_TRAINING_AUX_RD_INTERVAL values for DP 1.4

Manasi Navare
2018-03-07T01:36:36Z

On Wed, Mar 07, 2018 at 12:24:46AM +0000, Pandiyan, Dhinakaran wrote:
>
>
>
> On Tue, 2018-03-06 at 15:24 -0800, Rodrigo Vivi wrote:
> > On Tue, Mar 06, 2018 at 10:37:48AM -0800, matthew.s.atwood@intel.com wrote:
> > > From: Matt Atwood <matthew.s.atwood@intel.com>
> > >
> > > DP_TRAINING_AUX_RD_INTERVAL with DP 1.3 spec changed bit scheme from 8
> > > bits to 7 bits in DPCD 0x000e. The 8th bit describes a new feature, for
> > > panels that use this new feature, this would cause a wait interval for
> > > clock recovery of at least 512 ms, much higher then spec maximum of 16 ms.
> > > This behavior is described in table 2-158 of DP 1.4 spec address 0000Eh.
> > > To avoid breaking panels
>
> See comment below:
>
> > that are not spec compliant we now warn on
> > > invalid values.
> > >
> > > V2: commit title/message, masking all 7 bits, warn on out of spec values.
> >
> > this approach is even better imho.
> >
> > >
> > > Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
> >
> > Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> >
> > > ---
> > > drivers/gpu/drm/drm_dp_helper.c | 18 ++++++++++++++----
> > > include/drm/drm_dp_helper.h | 1 +
> > > 2 files changed, 15 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> > > index adf79be..a718ccc 100644
> > > --- a/drivers/gpu/drm/drm_dp_helper.c
> > > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > > @@ -119,18 +119,28 @@ u8 drm_dp_get_adjust_request_pre_emphasis(const u8 link_status[DP_LINK_STATUS_SI
> > > EXPORT_SYMBOL(drm_dp_get_adjust_request_pre_emphasis);
> > >
> > > void drm_dp_link_train_clock_recovery_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
> > > - if (dpcd[DP_TRAINING_AUX_RD_INTERVAL] == 0)
> > > + int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] & DP_TRAINING_AUX_RD_MASK;
> > > +
> > > + if (rd_interval > 4)
> > > + DRM_DEBUG_KMS("AUX interval %d, out of range (max 4)", rd_interval);
>
> Some default for panels without a valid value?
> rd_interval = 4;
> "AUX read interval out of range, using max %d ms"
>

The problem with setting the upper bound to 4 is that there are panels
that do not follow the spec and expect a longer than 16 ms delay. So
if we set the upper bound to 4 in those cases the panels might not work.

So we decided to go with this approach where we tell the users that panel is requesting
out of range AUX value but then set it to the value * 4 in the else part.

Manasi

>
> > > +
> > > + if (rd_interval == 0)
> > > udelay(100);
> > > else
> > > - mdelay(dpcd[DP_TRAINING_AUX_RD_INTERVAL] * 4);
> > > + mdelay(rd_interval * 4);
> > > }
> > > EXPORT_SYMBOL(drm_dp_link_train_clock_recovery_delay);
> > >
> > > void drm_dp_link_train_channel_eq_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
> > > - if (dpcd[DP_TRAINING_AUX_RD_INTERVAL] == 0)
> > > + int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] & DP_TRAINING_AUX_RD_MASK;
> > > +
> > > + if (rd_interval > 4)
> > > + DRM_DEBUG_KMS("AUX interval %d, out of range (max 4)", rd_interval);
> > > +
> > > + if (rd_interval == 0)
> > > udelay(400);
> > > else
> > > - mdelay(dpcd[DP_TRAINING_AUX_RD_INTERVAL] * 4);
> > > + mdelay(rd_interval * 4);
> > > }
> > > EXPORT_SYMBOL(drm_dp_link_train_channel_eq_delay);
> > >
> > > diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> > > index da58a42..f80acf1 100644
> > > --- a/include/drm/drm_dp_helper.h
> > > +++ b/include/drm/drm_dp_helper.h
> > > @@ -118,6 +118,7 @@
> > > # define DP_DPCD_DISPLAY_CONTROL_CAPABLE (1 << 3) /* edp v1.2 or higher */
> > >
> > > #define DP_TRAINING_AUX_RD_INTERVAL 0x00e /* XXX 1.2? */
> > > +# define DP_TRAINING_AUX_RD_MASK 0x7F /* 1.3 */
> > >
> > > #define DP_ADAPTER_CAP 0x00f /* 1.2 */
> > > # define DP_FORCE_LOAD_SENSE_CAP (1 << 0)
> > > --
> > > 2.7.4
> > >
> > > _______________________________________________
> > > Intel-gfx mailing list
> > > Intel-gfx@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> > _______________________________________________
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> > Intel-gfx@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
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