drm/dp_helper: Retry aux transactions on all errors
This is part of a patch series to migrate all of the workarounds for
commonly seen behavior from bad sinks in intel_dp_dpcd_read_wake() to
drm's DP helper.
We cannot rely on sinks NACKing or deferring when they can't receive
transactions, nor can we rely on any other sort of consistent error to
know when we should stop retrying. As such, we need to just retry
unconditionally on errors. We also make sure here to return the error we
encountered during the first transaction, since it's possible that
retrying the transaction might return a different error then we had
originally.
This, along with the previous patch, work around a weird bug with the
ThinkPad T560's and it's dock. When resuming the laptop, it appears that
there's a short period of time where we're unable to complete any aux
transactions, as they all immediately timeout. The only machine I'm able
to reproduce this on is the T560 as other production Skylake models seem
to be fine. The period during which AUX transactions fail appears to be
around 22ms long. AFAIK, the dock for the T560 never actually turns off,
the only difference is that it's in SST mode at the start of the resume
process, so it's unclear as to why it would need so much time to come
back up.
There's been a discussion on this issue going on for a while on the
intel-gfx mailing list about this that has, in addition to including
developers from Intel, also had the correspondence of one of the
hardware engineers for Intel:
http://www.spinics.net/lists/intel-gfx/msg88831.html
http://www.spinics.net/lists/intel-gfx/msg88410.html
We've already looked into a couple of possible explanations for the
problem:
- Calling intel_dp_mst_resume() before right fix.
intel_runtime_pm_enable_interrupts(). This was the first fix I tried,
and while it worked it definitely wasn't the right fix. This worked
because DP aux transactions don't actually require interrupts to work:
static uint32_t
intel_dp_aux_wait_done(struct intel_dp *intel_dp, bool has_aux_irq)
{
struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
struct drm_device *dev = intel_dig_port->base.base.dev;
struct drm_i915_private *dev_priv = dev->dev_private;
i915_reg_t ch_ctl = intel_dp->aux_ch_ctl_reg;
uint32_t status;
bool done;
#define C (((status = I915_READ_NOTRACE(ch_ctl)) & DP_AUX_CH_CTL_SEND_BUSY) == 0)
if (has_aux_irq)
done = wait_event_timeout(dev_priv->gmbus_wait_queue, C,
msecs_to_jiffies_timeout(10));
else
done = wait_for_atomic(C, 10) == 0;
if (!done)
DRM_ERROR("dp aux hw did not signal timeout (has irq: %i)!\n",
has_aux_irq);
#undef C
return status;
}
When there's no interrupts enabled, we end up timing out on the
wait_event_timeout() call, which causes us to check the DP status
register once to see if the transaction was successful or not. Since
this adds a 10ms delay to each aux transaction, it ends up adding a
long enough delay to the resume process for aux transactions to become
functional again. This gave us the illusion that enabling interrupts
had something to do with making things work again, and put me on the
wrong track for a while.
- Interrupts occurring when we try to perform the aux transactions
required to put the dock back into MST mode. This isn't the problem,
as the only interrupts I've observed that come during this timeout
period are from the snd_hda_intel driver, and disabling that driver
doesn't appear to change the behavior at all.
- Skylake's PSR block causing issues by performing aux transactions
while we try to bring the dock out of MST mode. Disabling PSR through
i915's command line options doesn't seem to change the behavior
either, nor does preventing the DMC firmware from being loaded.
Since this investigation went on for about 2 weeks, we decided it would
be better for the time being to just workaround this issue by making
sure AUX transactions wait a short period of time before retrying.
Signed-off-by: Lyude <cpaul@redhat.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460559513-32280-3-git-send-email-cpaul@redhat.com
1 file changed