Re: [PATCH v1 2/3] drm/msm/dpu: retrieve DSI DSC struct at atomic_check()

From: Kuogee Hsieh
Date: Wed May 31 2023 - 13:29:49 EST

On 5/31/2023 10:12 AM, Dmitry Baryshkov wrote:
On Wed, 31 May 2023 at 18:41, Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx> wrote:

+ if (dpu_enc->disp_info.intf_type == DRM_MODE_ENCODER_DSI) {

+ struct drm_bridge *bridge;
+ if (!dpu_enc->dsc) {
This condition is not correct. We should be updating the DSC even if
there is one.

+ bridge = drm_bridge_chain_get_first_bridge(drm_enc);
+ dpu_enc->dsc = msm_dsi_bridge_get_dsc_config(bridge);
This approach will not work for the hot-pluggable outputs. The dpu_enc
is not a part of the state. It should not be touched before
atomic_commit actually commits changes.
where can drm_dsc_config be stored?
I'd say, get it during atomic_check (and don't store it anywhere).
Then get it during atomic_enable (and save in dpu_enc).
got it.

Also, I don't think I like the API. It makes it impossible for the
driver to check that the bridge is the actually our DSI bridge or not.
Once you add DP here, the code will explode.

I think instead we should extend the drm_bridge API to be able to get
the DSC configuration from it directly. Additional care should be put
to design an assymetrical API. Theoretically a drm_bridge can be both
DSC source and DSC sink. Imagine a DSI-to-DP or DSI-to-HDMI bridge,
supporting DSC on the DSI side too.
Form my understanding, a bridge contains two interfaces.

Therefore I would think only one bridge for dsi-to-dp bridge? and this
bridge should represent the bridge chip?

I am thinking adding an ops function, get_bridge_dsc() to struct
drm_bridge_funcs to retrieve drm_dsc_config.
So, for this DSI-to-DP bridge will get_bridge_dsc() return DSC
configuration for the DSI or for the DP side of the bridge?

I would think should be DP side. there is no reason to enable dsc on both DSI and DP fro a bridge chip.

drm_bridge_chain_get_first_bridge(drm_enc) should return the bridge of the controller.

In DSI-to-DP bridge chip case, this controller will be the bridge chip who configured to perform protocol conversion between DSI and DP.

If DSC enabled should be at DP size which connect to panel.

Do you have other suggestion?
Let me think about it for a few days.