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 --- drivers/gpu/drm/drm_dp_helper.c | 39 +++++++++++++++++++++------------------ 1 file changed, 21 insertions(+), 18 deletions(-)
diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c index 3b915e2..86656ca 100644 --- a/drivers/gpu/drm/drm_dp_helper.c +++ b/drivers/gpu/drm/drm_dp_helper.c @@ -178,8 +178,8 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request, unsigned int offset, void *buffer, size_t size) { struct drm_dp_aux_msg msg; - unsigned int retry; - int err = 0; + unsigned int retry, native_reply; + int err = 0, ret = 0;
memset(&msg, 0, sizeof(msg)); msg.address = offset; @@ -194,34 +194,37 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request, * sufficient, bump to 32 which makes Dell 4k monitors happier. */ for (retry = 0; retry < 32; retry++) { - if (err != 0 && err != -ETIMEDOUT) { + if (ret != 0 && ret != -ETIMEDOUT) { usleep_range(AUX_RETRY_INTERVAL, AUX_RETRY_INTERVAL + 100); }
mutex_lock(&aux->hw_mutex); - err = aux->transfer(aux, &msg); + ret = aux->transfer(aux, &msg); mutex_unlock(&aux->hw_mutex); - if (err < 0) { - if (err == -EBUSY) - continue;
- return err; - } - - switch (msg.reply & DP_AUX_NATIVE_REPLY_MASK) { - case DP_AUX_NATIVE_REPLY_ACK: - if (err < size) - return -EPROTO; - return err; + if (ret > 0) { + native_reply = msg.reply & DP_AUX_NATIVE_REPLY_MASK; + if (native_reply == DP_AUX_NATIVE_REPLY_ACK) { + if (ret == size) + return ret;
- case DP_AUX_NATIVE_REPLY_NACK: - return -EIO; + ret = -EPROTO; + } else + ret = -EIO; } + + /* + * We want the error we return to be the error we received on + * the first transaction, since we may get a different error the + * next time we retry + */ + if (!err) + err = ret; }
DRM_DEBUG_KMS("too many retries, giving up\n"); - return -EIO; + return err; }
/**