On Thu, 26 Oct 2023 at 14:53, Maxime Ripard mripard@kernel.org wrote:
On Thu, Oct 26, 2023 at 11:57:22AM +0300, Dmitry Baryshkov wrote:
On Thu, 26 Oct 2023 at 11:07, Maxime Ripard mripard@kernel.org wrote:
On Thu, Oct 26, 2023 at 01:23:53AM +0300, Dmitry Baryshkov wrote:
+static int starfive_hdmi_register(struct drm_device *drm, struct starfive_hdmi *hdmi) +{
- struct drm_encoder *encoder = &hdmi->encoder;
- struct device *dev = hdmi->dev;
- encoder->possible_crtcs = drm_of_find_possible_crtcs(drm, dev->of_node);
- /*
- If we failed to find the CRTC(s) which this encoder is
- supposed to be connected to, it's because the CRTC has
- not been registered yet. Defer probing, and hope that
- the required CRTC is added later.
- */
- if (encoder->possible_crtcs == 0)
return -EPROBE_DEFER;- drm_encoder_helper_add(encoder, &starfive_hdmi_encoder_helper_funcs);
- hdmi->connector.polled = DRM_CONNECTOR_POLL_HPD;
- drm_connector_helper_add(&hdmi->connector,
&starfive_hdmi_connector_helper_funcs);- drmm_connector_init(drm, &hdmi->connector,
&starfive_hdmi_connector_funcs,DRM_MODE_CONNECTOR_HDMIA,On an embedded device one can not be so sure. There can be MHL or HDMI Alternative Mode. Usually we use drm_bridge here and drm_bridge_connector.
On an HDMI driver, it's far from being a requirement, especially given the limitations bridges have.
It's a blessing that things like MHL / HDMI-in-USB-C / HDMI-to-MyDP are not widely used in the wild and are mostly non-existing except several phones that preate wide DP usage.
And those can be supported without relying on bridges.
Yes, they likely can, in the way that nouveau handles I2C TV encoders. But I don't think this can scale. We can have different devices attached to the DSI, LVDS, HDMI and even DP image sources. I don't see a scalable solution for either of them. E.g. by switching drm/msm to use panel bridges for DSI panels we were able to significantly unify and simplify code paths.
Using drm_connector directly prevents one from handling possible modifications on the board level. For example, with the DRM connector in place, handling a separate HPD GPIO will result in code duplication from the hdmi-connector driver. Handling any other variations in the board design (which are pretty common in the embedded world) will also require changing the driver itself. drm_bridge / drm_bridge_connector save us from those issues.
And we have other solutions there too. Like, EDIDs are pretty much in the same spot with a lot of device variations, but it also works without a common driver. I'd really wish we were having less bridges and more helpers, but here we are.
BTW: what are the limitations of the drm_bridge wrt. HDMI output? I'm asking because we heavily depend on the bridge infrastructure for HDMI output. Maybe we are missing something there, which went unnoticed to me and my colleagues.
A bridge cannot extend the connector state or use properties, for example. It works for basic stuff but falls apart as soon as you're trying to do something slightly advanced.
Ack. I agree, we didn't have a necessity to implement properties up to now. But that sounds like an interesting topic for DSI-to-HDMI bridges and HDCP support. I'll need to check if any of the RB3/RB5/Dragonboard bridges are programmed with the HDCP keys. -- With best wishes Dmitry