Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad

From: Kuogee Hsieh
Date: Wed Apr 27 2022 - 17:24:46 EST

Tested-by: Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx>

On 4/26/2022 2:17 PM, Doug Anderson wrote:

On Tue, Apr 26, 2022 at 2:11 PM Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> wrote:

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

On Tue, Apr 26, 2022 at 12:20 PM Abhinav Kumar
<quic_abhinavk@xxxxxxxxxxx> 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.


On 4/26/2022 11:46 AM, Douglas Anderson wrote:
As per Displayport spec section ("Video Timing Format") says
that all detachable sinks shall support 640x480 @60Hz as a fail safe

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"

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


Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

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
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 =
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->, 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,
- drm_mode_prune_invalid(dev, &connector->modes, verbose_prune);
+ drm_mode_prune_invalid(dev, &connector->modes, true);
+ /*
+ * Displayport spec section ("Video Timing Format") says
+ * 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
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
This will help cover the case of but not fix

For, 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 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. will still be broken.

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