Re: [PATCH v6 3/8] drm/bridge: mhdp8546: Add minimal format negotiation

From: Tomi Valkeinen
Date: Fri May 26 2023 - 05:30:08 EST

On 16/05/2023 17:25, Aradhya Bhatia wrote:
Hi Neil,

Thank you for reviewing the patch.

On 16-May-23 12:51, Neil Armstrong wrote:
On 15/05/2023 17:59, Aradhya Bhatia wrote:
Hi Tomi,

On 12-May-23 14:45, Tomi Valkeinen wrote:
On 09/05/2023 12:30, Aradhya Bhatia wrote:
From: Nikhil Devshatwar <nikhil.nd@xxxxxx>

With new connector model, mhdp bridge will not create the connector and
SoC driver will rely on format negotiation to setup the encoder format.

Support minimal format negotiations hooks in the drm_bridge_funcs.
Complete format negotiation can be added based on EDID data.
This patch adds the minimal required support to avoid failure
after moving to new connector model.

Signed-off-by: Nikhil Devshatwar <nikhil.nd@xxxxxx>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@xxxxxx>

You need to add your SoB to this and the other patches.




      changes from v1:
      * cosmetic fixes, commit message update.

      changes from v5:
      * dropped the default_bus_format variable and directly assigned
        MEDIA_BUS_FMT_RGB121212_1X36 to input_fmts.

   .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 25
   1 file changed, 25 insertions(+)

diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
index f6822dfa3805..623e4235c94f 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
+++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
@@ -2146,6 +2146,30 @@ cdns_mhdp_bridge_atomic_reset(struct drm_bridge
       return &cdns_mhdp_state->base;
   +static u32 *cdns_mhdp_get_input_bus_fmts(struct drm_bridge *bridge,
+                     struct drm_bridge_state *bridge_state,
+                     struct drm_crtc_state *crtc_state,
+                     struct drm_connector_state *conn_state,
+                     u32 output_fmt,
+                     unsigned int *num_input_fmts)
+    u32 *input_fmts;
+    *num_input_fmts = 0;
+    if (output_fmt != MEDIA_BUS_FMT_FIXED)
+        return NULL;

The tfp410 and sii902x drivers don't have the above check. Why does mhdp
need it? Or the other way, why don't tfp410 and sii902x need it?

I had removed this condition in order to follow status quo, from the
ITE-66121 HDMI bridge driver.

The idea would have been to drop this for MHDP as well, but I guess I
overlooked this one.


I guess at the moment we always do get MEDIA_BUS_FMT_FIXED as the out
fmt (in all three bridge drivers), don't we?

... I tested again to ensure that the above is indeed the case. And
ended up catching some odd behavior.

It turns out that for all the HDMI bridges (TFP410, SII902X, ITE-66121),
the format negotiation doesn't stop at output_fmt = MEDIA_BUS_FMT_FIXED.
The {bridge}_get_input_format API gets called again with the output_fmt

This doesn't happen with the MHDP driver. Format negotiation with MHDP
bridge stops after one round, at output_fmt = MEDIA_BUS_FMT_FIXED.

This is because the bridge negociation logic will test with all possible
output formats from the chain, and won't stop at first working test.


If your bridge only supports a single input format, it should return the
same format whatever output_fmt is tried.

So indeed remove this test on mhdp aswell, or filter out invalid output

I have been looking into the code deeper and trying to understand the
logic flow around the format negotiation in the framework. Here are the
2 points that I want to mention. Please let me know if I have missed
something with my understanding.

Firstly, the mhdp-8546 output connects to the display-connector (with
the compatible, "dp-connector") in the devicetree.

When the negotiation begins at 'drm_atomic_bridge_chain_select_bus_fmts'
the display-connector bridge *should* act as the 'last_bridge', and the
atomic_get_output_bus_fmts hook of the display-connector should get
called. However, for some reason I am not yet sure of, the condition

:: if (last_bridge->funcs->atomic_get_output_bus_fmts)

fails and the 'select_bus_fmt_recursive' function gets called instead,
(with MEDIA_BUS_FMT_FIXED as output_fmt), which in turn calls the
atomic_get_input_bus_fmts hook of the mhdp-8546. This entirely skips the
display-connector out of the format negotiation.

This doesn't happen when the HDMI bridges are in concern, even though,
they too are connected with display-connector (with compatible

I looked into the display-connector driver hoping to find if the 2 types
of connectors are being treated differently wrt format negotiation, but
I did not find any clue.

Please let me know if you have any idea about this.

The display connector is probed, but not attached to the bridge chain, so the last bridge is the mdhp. You need to call drm_bridge_attach in the mhdp driver to attach the next bridge. See e.g. tfp410's tfp410_attach().

Also, I think the support in mhdp for the !DRM_BRIDGE_ATTACH_NO_CONNECTOR case should be dropped.

Secondly, as mentioned in the display-connector driver, this bridge is
essentially a pass-through. And hence to reflect that, both the format
negotiation hooks of display-connector driver call their counter-parts
from the previous bridge if they are available, and if not, the formats
are assigned MEDIA_BUS_FMT_FIXED.

While this makes sense for the atomic_get_output_bus_fmts hook, it seems
to me that, the same may not hold true for the atomic_get_input_bus_fmts
If the bridge is indeed a pass-through, should it not also pass the
output_format as its input format (which it actually got from the output
of previous bridge)?

This way all the following will remain same.

1. output_fmt of prev_bridge,
2. input_fmt of display-connector, and
3. output_fmt of display-connector.

Currently, since the atomic_get_input_bus_fmts hook of display-connector
calls its counter-part from the prev_bridge, the input_fmt it passes
(for HDMI bridges) is MEDIA_BUS_FMT_RGB888_1X24. The
atomic_get_ouput_bus_fmts hook of the HDMI bridge has to, then, set an
input bus format considering MEDIA_BUS_FMT_RGB888_1X24 as its output

Let me know what you think!

Yes, it does sound odd to me. Returning the same from the display-connector's get-input-fmt and get-output-fmt makes more sense.

Btw, we seem to be missing get-output-fmt from the mdhp driver.