At Wed, 04 Dec 2013 12:17:03 +0200,
Jani Nikula wrote:
>
> On Fri, 22 Nov 2013, Takashi Iwai <tiwai@xxxxxxx> wrote:
> > I got kernel WARNINGs frequently on Haswell laptops complaining about
> > invalid max DP link bw. With drm.debug=0x0e, it turned out that the
> > obtained DPCD is utterly bogus when it happens:
> > [drm:intel_dp_get_dpcd], DPCD: 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d
> >
> > This seems happening intermittently without plug state changes and
> > even after the first few reads succeeded. As a workaround, add a
> > sanity check for such bogus values and retry reading if it hits.
>
> If it's not related to crtc disable/enable state changes, then I assume
> this might happen with any native aux reads at any time, and your patch
> would just paper over one instance of it.
Yes, exactly, that's the purpose :)
> Does this occur with several different displays? With the displays this
> occurs with, does it also occur on non-Haswell?
I haven't seen on IvyBridge machines, at least.
And I have no other DP monitor, so cannot test right now.
> Have you observed that you'd ever need to try again more than once?
> I.e. does the first retry sometimes return garbage too?
I don't remember whether more than once were needed.
Takashi
> Instinctively I'd say we need to resist the urge to apply this
> workaround, and try to find the root cause.
>
> BR,
> Jani.
>
> >
> > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > ---
> >
> > drivers/gpu/drm/i915/intel_dp.c | 15 ++++++++++++++-
> > 1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index eb8139da9763..f596353da557 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -2792,13 +2792,26 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
> > struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
> > struct drm_device *dev = dig_port->base.base.dev;
> > struct drm_i915_private *dev_priv = dev->dev_private;
> > -
> > + int retry = 0;
> > char dpcd_hex_dump[sizeof(intel_dp->dpcd) * 3];
> >
> > + again:
> > if (intel_dp_aux_native_read_retry(intel_dp, 0x000, intel_dp->dpcd,
> > sizeof(intel_dp->dpcd)) == 0)
> > return false; /* aux transfer failed */
> >
> > + /* sanity check: sometimes DPCD contains a series of same bogus value
> > + * even though the above returned success. Retry reading in that case.
> > + */
> > + if (intel_dp->dpcd[0]) {
> > + int i;
> > + for (i = 1; i < sizeof(intel_dp->dpcd); i++)
> > + if (intel_dp->dpcd[i] != intel_dp->dpcd[0])
> > + break;
> > + if (i >= sizeof(intel_dp->dpcd) && retry++ < 2)
> > + goto again;
> > + }
> > +
> > hex_dump_to_buffer(intel_dp->dpcd, sizeof(intel_dp->dpcd),
> > 32, 1, dpcd_hex_dump, sizeof(dpcd_hex_dump), false);
> > DRM_DEBUG_KMS("DPCD: %s\n", dpcd_hex_dump);
> > --
> > 1.8.4.3
> >
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>
> --
> Jani Nikula, Intel Open Source Technology Center
>
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