Hi Doug

One minor comment below.

But otherwise, looking at this change this should work for us acc to me.

We will test this out with our equipment and then provide R-b.

Thanks

Abhinav On 4/26/2022 11:46 AM, Douglas Anderson wrote:

As per Displayport spec section 5.2.1.2 ("Video Timing Format") says that all detachable sinks shall support 640x480 @60Hz as a fail safe mode.

A DP compliance test expected us to utilize the above fact when all modes it presented to the DP source were not achievable. It presented only modes that would be achievable with more lanes and/or higher speeds than we had available and expected that when we couldn't do that then we'd fall back to 640x480 even though it didn't advertise this size.

In order to pass the compliance test (and also support any users who might fall into a similar situation with their display), we need to add 640x480 into the list of modes. However, we don't want to add 640x480 all the time. Despite the fact that the DP spec says all sinks _shall support_ 640x480, they're not guaranteed to support it _well_. Continuing to read the spec you can see that the display is not required to really treat 640x480 equal to all the other modes. It doesn't need to scale or anything--just display the pixels somehow for failsafe purposes. It should also be noted that it's not hard to find a display hooked up via DisplayPort that _doesn't_ support 640x480 at all. The HP ZR30w screen I'm sitting in front of has a native DP port and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI display via a DP to HDMI adapter and that screen definitely doesn't support 640x480.

As a compromise solution, let's only add the 640x480 mode if:

• We're on DP.
• All other modes have been pruned.

This acknowledges that 640x480 might not be the best mode to use but, since sinks are _supposed_ to support it, we will at least fall back to it if there's nothing else.

Note that we _don't_ add higher resolution modes like 1024x768 in this case. We only add those modes for a failed EDID read where we have no idea what's going on. In the case where we've pruned all modes then instead we only want 640x480 which is the only defined "Fail Safe" resolution.

This patch originated in response to Kuogee Hsieh's patch [1].

[1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quic...

Signed-off-by: Douglas Anderson dianders@chromium.org

drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c index 819225629010..90cd46cbfec1 100644 --- a/drivers/gpu/drm/drm_probe_helper.c +++ b/drivers/gpu/drm/drm_probe_helper.c @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, const struct drm_connector_helper_funcs *connector_funcs = connector->helper_private; int count = 0, ret;

• bool verbose_prune = true; enum drm_connector_status old_status; struct drm_modeset_acquire_ctx ctx;

@@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n", connector->base.id, connector->name); drm_connector_update_edid_property(connector, NULL);

• verbose_prune = false;
  

• goto prune;
  

• drm_mode_prune_invalid(dev, &connector->modes, false);
  

• goto exit;
  

  }

  count = (*connector_funcs->get_modes)(connector);

@@ -580,9 +579,26 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, } }

-prune:

• drm_mode_prune_invalid(dev, &connector->modes, verbose_prune);

• drm_mode_prune_invalid(dev, &connector->modes, true);

• /*

• * Displayport spec section 5.2.1.2 ("Video Timing Format") says that
  

• * all detachable sinks shall support 640x480 @60Hz as a fail safe
  

• * mode. If all modes were pruned, perhaps because they need more
  

• * lanes or a higher pixel clock than available, at least try to add
  

• * in 640x480.
  

• */
  

• if (list_empty(&connector->modes) &&

•    connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) {
  

• count = drm_add_modes_noedid(connector, 640, 480);
  

• if (_drm_helper_update_and_validate(connector, maxX, maxY, &ctx)) {
  

• 	drm_modeset_backoff(&ctx);
  

• 	goto retry;
  

Do we need another retry here? This will again repeat everything from get_modes(). The fact that we are hitting this code is because we have already tried that and this is already a second-pass. So I think another retry isnt needed?

• }
  

• drm_mode_prune_invalid(dev, &connector->modes, true);
  

• }

+exit: drm_modeset_drop_locks(&ctx); drm_modeset_acquire_fini(&ctx);

Hi,

On Tue, Apr 26, 2022 at 12:20 PM Abhinav Kumar quic_abhinavk@quicinc.com wrote:

Missed one more comment.

On 4/26/2022 12:16 PM, Abhinav Kumar wrote:

...

Hi Doug

One minor comment below.

But otherwise, looking at this change this should work for us acc to me.

We will test this out with our equipment and then provide R-b.

Thanks

Abhinav On 4/26/2022 11:46 AM, Douglas Anderson wrote:

...

As per Displayport spec section 5.2.1.2 ("Video Timing Format") says that all detachable sinks shall support 640x480 @60Hz as a fail safe mode.

A DP compliance test expected us to utilize the above fact when all modes it presented to the DP source were not achievable. It presented only modes that would be achievable with more lanes and/or higher speeds than we had available and expected that when we couldn't do that then we'd fall back to 640x480 even though it didn't advertise this size.

In order to pass the compliance test (and also support any users who might fall into a similar situation with their display), we need to add 640x480 into the list of modes. However, we don't want to add 640x480 all the time. Despite the fact that the DP spec says all sinks _shall support_ 640x480, they're not guaranteed to support it _well_. Continuing to read the spec you can see that the display is not required to really treat 640x480 equal to all the other modes. It doesn't need to scale or anything--just display the pixels somehow for failsafe purposes. It should also be noted that it's not hard to find a display hooked up via DisplayPort that _doesn't_ support 640x480 at all. The HP ZR30w screen I'm sitting in front of has a native DP port and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI display via a DP to HDMI adapter and that screen definitely doesn't support 640x480.

As a compromise solution, let's only add the 640x480 mode if:

• We're on DP.
• All other modes have been pruned.

This acknowledges that 640x480 might not be the best mode to use but, since sinks are _supposed_ to support it, we will at least fall back to it if there's nothing else.

Note that we _don't_ add higher resolution modes like 1024x768 in this case. We only add those modes for a failed EDID read where we have no idea what's going on. In the case where we've pruned all modes then instead we only want 640x480 which is the only defined "Fail Safe" resolution.

This patch originated in response to Kuogee Hsieh's patch [1].

[1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quic...

Signed-off-by: Douglas Anderson dianders@chromium.org

drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c index 819225629010..90cd46cbfec1 100644 --- a/drivers/gpu/drm/drm_probe_helper.c +++ b/drivers/gpu/drm/drm_probe_helper.c @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, const struct drm_connector_helper_funcs *connector_funcs = connector->helper_private; int count = 0, ret;

• bool verbose_prune = true; enum drm_connector_status old_status; struct drm_modeset_acquire_ctx ctx;

@@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n", connector->base.id, connector->name); drm_connector_update_edid_property(connector, NULL);

•    verbose_prune = false;
  

•    goto prune;
  

•    drm_mode_prune_invalid(dev, &connector->modes, false);
  

•    goto exit;
  }
  count = (*connector_funcs->get_modes)(connector);
  

@@ -580,9 +579,26 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, } } -prune:

• drm_mode_prune_invalid(dev, &connector->modes, verbose_prune);

• drm_mode_prune_invalid(dev, &connector->modes, true);
• /*

• * Displayport spec section 5.2.1.2 ("Video Timing Format") says
  

that

• * all detachable sinks shall support 640x480 @60Hz as a fail safe
  

• * mode. If all modes were pruned, perhaps because they need more
  

• * lanes or a higher pixel clock than available, at least try to add
  

• * in 640x480.
  

• */
  

• if (list_empty(&connector->modes) &&

•    connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) {
  

•    count = drm_add_modes_noedid(connector, 640, 480);
  

•    if (_drm_helper_update_and_validate(connector, maxX, maxY,
  

&ctx)) {

•        drm_modeset_backoff(&ctx);
  

•        goto retry;
  

Do we need another retry here? This will again repeat everything from get_modes(). The fact that we are hitting this code is because we have already tried that and this is already a second-pass. So I think another retry isnt needed?

This will help cover the case of 4.2.2.6 but not fix 4.2.2.1.

For 4.2.2.1, we will have 0 modes and so the original DRM fwk code of adding all modes <= 1024x768 will kick in.

Now, in that list, we will still need to pick/mark 640x480 as the preferred mode.

We still need IGT for that.

Are you sure you don't have those backwards? It seems like 4.2.2.6 is the test case dealing with corrupt EDID and that's the one that will still be broken, no? ...and corrupt EDID is still the case where we have 0 modes.

In any case, let's see what people think about:

https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f...

I've marked that one as RFC just because it seems like a bigger change to existing behavior, though it still seems correct to me.

NOTE: reading 4.2.2.6 more closely, it actually looks as if we're actually supposed to be able to try various video modes one at a time until we find one that works (or land on 640x480). Seems as if we're supposed to be able to try the higher resolutions one at a time and we can tell whether the sink "accepted" it by seeing if SINK_STATUS goes to 1? I have no idea how that works with all the Linux APIs, though.

-Doug

Hi,

On Tue, Apr 26, 2022 at 2:11 PM Abhinav Kumar quic_abhinavk@quicinc.com wrote:

On 4/26/2022 1:26 PM, Doug Anderson wrote:

...

Hi,

On Tue, Apr 26, 2022 at 12:20 PM Abhinav Kumar quic_abhinavk@quicinc.com wrote:

...

Missed one more comment.

On 4/26/2022 12:16 PM, Abhinav Kumar wrote:

...

Hi Doug

One minor comment below.

But otherwise, looking at this change this should work for us acc to me.

We will test this out with our equipment and then provide R-b.

Thanks

Abhinav On 4/26/2022 11:46 AM, Douglas Anderson wrote:

...

As per Displayport spec section 5.2.1.2 ("Video Timing Format") says that all detachable sinks shall support 640x480 @60Hz as a fail safe mode.

A DP compliance test expected us to utilize the above fact when all modes it presented to the DP source were not achievable. It presented only modes that would be achievable with more lanes and/or higher speeds than we had available and expected that when we couldn't do that then we'd fall back to 640x480 even though it didn't advertise this size.

In order to pass the compliance test (and also support any users who might fall into a similar situation with their display), we need to add 640x480 into the list of modes. However, we don't want to add 640x480 all the time. Despite the fact that the DP spec says all sinks _shall support_ 640x480, they're not guaranteed to support it _well_. Continuing to read the spec you can see that the display is not required to really treat 640x480 equal to all the other modes. It doesn't need to scale or anything--just display the pixels somehow for failsafe purposes. It should also be noted that it's not hard to find a display hooked up via DisplayPort that _doesn't_ support 640x480 at all. The HP ZR30w screen I'm sitting in front of has a native DP port and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI display via a DP to HDMI adapter and that screen definitely doesn't support 640x480.

As a compromise solution, let's only add the 640x480 mode if:

• We're on DP.
• All other modes have been pruned.

This acknowledges that 640x480 might not be the best mode to use but, since sinks are _supposed_ to support it, we will at least fall back to it if there's nothing else.

Note that we _don't_ add higher resolution modes like 1024x768 in this case. We only add those modes for a failed EDID read where we have no idea what's going on. In the case where we've pruned all modes then instead we only want 640x480 which is the only defined "Fail Safe" resolution.

This patch originated in response to Kuogee Hsieh's patch [1].

[1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quic...

Signed-off-by: Douglas Anderson dianders@chromium.org

drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c index 819225629010..90cd46cbfec1 100644 --- a/drivers/gpu/drm/drm_probe_helper.c +++ b/drivers/gpu/drm/drm_probe_helper.c @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, const struct drm_connector_helper_funcs *connector_funcs = connector->helper_private; int count = 0, ret;

• bool verbose_prune = true; enum drm_connector_status old_status; struct drm_modeset_acquire_ctx ctx;

@@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n", connector->base.id, connector->name); drm_connector_update_edid_property(connector, NULL);

•    verbose_prune = false;
  

•    goto prune;
  

•    drm_mode_prune_invalid(dev, &connector->modes, false);
  

•    goto exit;
   }
   count = (*connector_funcs->get_modes)(connector);
  

@@ -580,9 +579,26 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, } } -prune:

• drm_mode_prune_invalid(dev, &connector->modes, verbose_prune);

• drm_mode_prune_invalid(dev, &connector->modes, true);
• /*

• * Displayport spec section 5.2.1.2 ("Video Timing Format") says
  

that

• * all detachable sinks shall support 640x480 @60Hz as a fail safe
  

• * mode. If all modes were pruned, perhaps because they need more
  

• * lanes or a higher pixel clock than available, at least try to add
  

• * in 640x480.
  

• */
  

• if (list_empty(&connector->modes) &&

•    connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) {
  

•    count = drm_add_modes_noedid(connector, 640, 480);
  

•    if (_drm_helper_update_and_validate(connector, maxX, maxY,
  

&ctx)) {

•        drm_modeset_backoff(&ctx);
  

•        goto retry;
  

Do we need another retry here? This will again repeat everything from get_modes(). The fact that we are hitting this code is because we have already tried that and this is already a second-pass. So I think another retry isnt needed?

This will help cover the case of 4.2.2.6 but not fix 4.2.2.1.

For 4.2.2.1, we will have 0 modes and so the original DRM fwk code of adding all modes <= 1024x768 will kick in.

Now, in that list, we will still need to pick/mark 640x480 as the preferred mode.

We still need IGT for that.

Are you sure you don't have those backwards? It seems like 4.2.2.6 is the test case dealing with corrupt EDID and that's the one that will still be broken, no? ...and corrupt EDID is still the case where we have 0 modes.

Yes indeed, sorry, I did have the numbers backwards. 4.2.2.6 will still be broken.

...

In any case, let's see what people think about:

https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f...

Yes sure. If it gets accepted, it will save us some IGT work.

...

I've marked that one as RFC just because it seems like a bigger change to existing behavior, though it still seems correct to me.

NOTE: reading 4.2.2.6 more closely, it actually looks as if we're actually supposed to be able to try various video modes one at a time until we find one that works (or land on 640x480). Seems as if we're supposed to be able to try the higher resolutions one at a time and we can tell whether the sink "accepted" it by seeing if SINK_STATUS goes to 1? I have no idea how that works with all the Linux APIs, though.

hmmm .... our equipment throws a warning if we dont sent 640x480. So perhaps just go with the "or land on 640x480" option.

0006.392.232: [WARNING] Source DUT failed to transmit a video stream using fail-safe mode 0006.392.491: Received 1344 Htotal differs from fail-safe 800 0006.392.621: Received 1024 Hactive differs from fail-safe 640 0006.392.750: Received 296 Hstart differs from fail-safe 144 0006.392.868: Received 136 Hsync width differs from fail-safe 96 0006.392.975: Received 806 Vtotal differs from fail-safe 525 0006.393.099: Received 768 Vactive differs from fail-safe 480 0006.393.229: Received 6 Vsync width differs from fail-safe 2

Do you actually have code to implement the checking of SINK_STATUS? I'm not aware of how that would work in Linux, which is why just defaulting to 640x480 seems like a reasonable thing to do for now. The test case actually says that you're allowed to try clock rates one at a time (polling SINK_STATUS in DPCT) as long as you don't spend more than 5 seconds on each clock rate. According to the test case if you never saw SINK_STATUS in DPCT go to 1 then you should end at 640x480.

Hi,

On Tue, Apr 26, 2022 at 12:16 PM Abhinav Kumar quic_abhinavk@quicinc.com wrote:

Hi Doug

One minor comment below.

But otherwise, looking at this change this should work for us acc to me.

We will test this out with our equipment and then provide R-b.

Thanks

Abhinav On 4/26/2022 11:46 AM, Douglas Anderson wrote:

...

As per Displayport spec section 5.2.1.2 ("Video Timing Format") says that all detachable sinks shall support 640x480 @60Hz as a fail safe mode.

A DP compliance test expected us to utilize the above fact when all modes it presented to the DP source were not achievable. It presented only modes that would be achievable with more lanes and/or higher speeds than we had available and expected that when we couldn't do that then we'd fall back to 640x480 even though it didn't advertise this size.

In order to pass the compliance test (and also support any users who might fall into a similar situation with their display), we need to add 640x480 into the list of modes. However, we don't want to add 640x480 all the time. Despite the fact that the DP spec says all sinks _shall support_ 640x480, they're not guaranteed to support it _well_. Continuing to read the spec you can see that the display is not required to really treat 640x480 equal to all the other modes. It doesn't need to scale or anything--just display the pixels somehow for failsafe purposes. It should also be noted that it's not hard to find a display hooked up via DisplayPort that _doesn't_ support 640x480 at all. The HP ZR30w screen I'm sitting in front of has a native DP port and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI display via a DP to HDMI adapter and that screen definitely doesn't support 640x480.

As a compromise solution, let's only add the 640x480 mode if:

• We're on DP.
• All other modes have been pruned.

This acknowledges that 640x480 might not be the best mode to use but, since sinks are _supposed_ to support it, we will at least fall back to it if there's nothing else.

Note that we _don't_ add higher resolution modes like 1024x768 in this case. We only add those modes for a failed EDID read where we have no idea what's going on. In the case where we've pruned all modes then instead we only want 640x480 which is the only defined "Fail Safe" resolution.

This patch originated in response to Kuogee Hsieh's patch [1].

[1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quic...

Signed-off-by: Douglas Anderson dianders@chromium.org

drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c index 819225629010..90cd46cbfec1 100644 --- a/drivers/gpu/drm/drm_probe_helper.c +++ b/drivers/gpu/drm/drm_probe_helper.c @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, const struct drm_connector_helper_funcs *connector_funcs = connector->helper_private; int count = 0, ret;

• bool verbose_prune = true;
  enum drm_connector_status old_status;
  struct drm_modeset_acquire_ctx ctx;
  
  

@@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n", connector->base.id, connector->name); drm_connector_update_edid_property(connector, NULL);

•         verbose_prune = false;
  

•         goto prune;
  

•         drm_mode_prune_invalid(dev, &connector->modes, false);
  

•         goto exit;
  }
  
  count = (*connector_funcs->get_modes)(connector);
  

@@ -580,9 +579,26 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, } }

-prune:

• drm_mode_prune_invalid(dev, &connector->modes, verbose_prune);
  

• drm_mode_prune_invalid(dev, &connector->modes, true);
  
  

• /*
  

•  * Displayport spec section 5.2.1.2 ("Video Timing Format") says that
  

•  * all detachable sinks shall support 640x480 @60Hz as a fail safe
  

•  * mode. If all modes were pruned, perhaps because they need more
  

•  * lanes or a higher pixel clock than available, at least try to add
  

•  * in 640x480.
  

•  */
  

• if (list_empty(&connector->modes) &&
  

•     connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) {
  

•         count = drm_add_modes_noedid(connector, 640, 480);
  

•         if (_drm_helper_update_and_validate(connector, maxX, maxY, &ctx)) {
  

•                 drm_modeset_backoff(&ctx);
  

•                 goto retry;
  

Do we need another retry here? This will again repeat everything from get_modes(). The fact that we are hitting this code is because we have already tried that and this is already a second-pass. So I think another retry isnt needed?

The retry is still needed. This gets into the whole wait-wound mutexes that DRM uses, right? Any time we detect deadlock we release all of our locks and start from scratch. That's still possible here.

-Doug

Hi,

On Thu, May 5, 2022 at 11:34 AM Thomas Zimmermann tzimmermann@suse.de wrote:

Hi

Am 26.04.22 um 20:46 schrieb Douglas Anderson:

...

The drm_helper_probe_single_connector_modes() is a bit long. Let's break a chunk off to update and validate modes. This helps avoid one goto and also will allow us to more easily call the helper a second time in a future patch without adding looping or another goto.

This change is intended to be a no-op change--just code movement.

Signed-off-by: Douglas Anderson dianders@chromium.org

...

---

drivers/gpu/drm/drm_probe_helper.c | 105 ++++++++++++++++------------- 1 file changed, 59 insertions(+), 46 deletions(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c index 682359512996..819225629010 100644 --- a/drivers/gpu/drm/drm_probe_helper.c +++ b/drivers/gpu/drm/drm_probe_helper.c @@ -354,6 +354,61 @@ drm_helper_probe_detect(struct drm_connector *connector, } EXPORT_SYMBOL(drm_helper_probe_detect);

+static bool _drm_helper_update_and_validate(struct drm_connector *connector,

AFAIK convention is to use two underscores for internal names.

Sure! I'll spin with this.

...

•                                     uint32_t maxX, uint32_t maxY,
  

•                                     struct drm_modeset_acquire_ctx *ctx)
  

+{

• struct drm_device *dev = connector->dev;
  

• struct drm_display_mode *mode;
  

• int mode_flags = 0;
  

• int ret;
  

• drm_connector_list_update(connector);
  

• if (connector->interlace_allowed)
  

•         mode_flags |= DRM_MODE_FLAG_INTERLACE;
  

• if (connector->doublescan_allowed)
  

•         mode_flags |= DRM_MODE_FLAG_DBLSCAN;
  

• if (connector->stereo_allowed)
  

•         mode_flags |= DRM_MODE_FLAG_3D_MASK;
  

• list_for_each_entry(mode, &connector->modes, head) {
  

•         if (mode->status != MODE_OK)
  

•                 continue;
  

•         mode->status = drm_mode_validate_driver(dev, mode);
  

•         if (mode->status != MODE_OK)
  

•                 continue;
  

•         mode->status = drm_mode_validate_size(mode, maxX, maxY);
  

•         if (mode->status != MODE_OK)
  

•                 continue;
  

•         mode->status = drm_mode_validate_flag(mode, mode_flags);
  

•         if (mode->status != MODE_OK)
  

•                 continue;
  

•         ret = drm_mode_validate_pipeline(mode, connector, ctx,
  

•                                          &mode->status);
  

•         if (ret) {
  

•                 drm_dbg_kms(dev,
  

•                             "drm_mode_validate_pipeline failed: %d\n",
  

•                             ret);
  

•                 if (drm_WARN_ON_ONCE(dev, ret != -EDEADLK))
  

•                         mode->status = MODE_ERROR;
  

•                 else
  

•                         return true;
  

Returning true is non-intuitive. It looks as if we report success when it actually signals a retry.

I suggest to return 'ret' here and let the caller decide. On success at the end of the function, it would return 0 as usual.

There's a madness to my method. I originally had it returning "ret" but then I felt like the callers now needed to handle three cases:

a) ret = -EDEADLK b) ret = 0 c) ret = some other error

In reality _drm_helper_update_and_validate() never returned anything other than a) or b), so adding the extra error handling for c) seemed like a waste. ...but it also felt like if it violated the abstraction of _drm_helper_update_and_validate() returning an error code if I didn't handle c).

In any case, I'll change it back to an error code. Maybe a compromise would be:

ret = _drm_helper_update_and_validate(...) if (ret == -EDEADLK) { drm_modeset_backoff(...) goto retry; } WARN_ON(ret);

...so we at least document that ret can only be one of the two values and we'll get a warning splat if it ever happens, but we don't need to add complex error handling for a case that the code can't hit. Ah, looking above I guess that matches what the function does earlier too.

OK, I'll give a few more days for feedback on this series and I'll spin with the two changes you've requested.

-Doug

-Doug