On Thu, Mar 24, 2022 at 03:43:42PM +0200, Laurent Pinchart wrote: > On Thu, Mar 24, 2022 at 09:18:19AM +0100, Maxime Ripard wrote: > > On Wed, Mar 23, 2022 at 10:38:19PM +0200, Laurent Pinchart wrote: > > > Hi Maxime, > > > > > > (CC'ing Sakari) > > > > > > Thank you for the patch. > > > > > > On Wed, Mar 23, 2022 at 04:48:23PM +0100, Maxime Ripard wrote: > > > > MIPI-DSI devices, if they are controlled through the bus itself, have to > > > > be described as a child node of the controller they are attached to. > > > > > > > > Thus, there's no requirement on the controller having an OF-Graph output > > > > port to model the data stream: it's assumed that it would go from the > > > > parent to the child. > > > > > > > > However, some bridges controlled through the DSI bus still require an > > > > input OF-Graph port, thus requiring a controller with an OF-Graph output > > > > port. This prevents those bridges from being used with the controllers > > > > that do not have one without any particular reason to. > > > > > > > > Let's drop that requirement. > > > > > > I'm sure this won't come as a surprise, I'm very much opposed to this > > > change, for two reasons. > > > > > > First, ports are part of the hardware, even if they're not connected. It > > > thus simplifies handling in drivers if they're always present. > > > > > > Then, and that's the most important reason, I think it's a mistake not > > > to model the DSI data connection using OF graph unconditionally, even > > > when the DSI sink device is also controlled through the DSI bus (using > > > DCS) and is in that case a child of the DSI source device in the DT > > > hierarchy. > > > > That's the way we do for any other device though. You never addressed > > that comment, but it's very much the same that occurs for i2c or spi > > controllers and their device. They all get their data from the parent > > bus. I don't see you advocate for using OF-Graph for those devices. > > Those are different, there's no data stream independent of the control > communications. Fine, then you have Ethernet PHYs, or any MMIO device that does DMA. > > > The device tree describes a control hierarchy between devices. OF graph > > > overlays on top of that a data transfer graph. The two are different > > > concepts, and the fact that DSI can sometimes be used as a control bus > > > doesn't change the concept. Using OF graph unconditionally to describe > > > the data connections for DSI leads to less variation in the device tree > > > structure, and thus less complexity in the implementation. We're > > > suffering from the fact we haven't made it a requirement in the first > > > place, which can't be fixed due to ABI breakage constraints, but let's > > > not acknowledge it as a good idea. > > > > Honestly, it doesn't matter one bit. > > > > We have a huge discrepancy here today, and only a couple of bridges have > > that arbitrary restriction. The situation you don't want to acknowledge > > is the de-facto standard, by the generic binding and by what all the > > bridges and panels are implementing. Even panel-simple-dsi is doing it. > > So it's very much there already. > > It's here, and I think we should move away from it for new DSI sinks. > I'd like OF graph to be used consistently for new drivers. We can't > change existing DT bindings and drivers to drop support for the > non-OF-graph description due to ABI stability, but we can avoid > repeating the mistake going forward. > > > What I'm trying to address here is that some controllers that do > > everything right can't be used because that restriction is completely > > arbitrary and in opposition to the consensus. And they can't be used > > *today*. > > > > If we want to change that consensus, fine, but we should still have one. > > Having some bridges enforcing custom rules for no reason is very much > > unacceptable. > > > > And changing that consensus won't happen overtime, we'll have to take > > care of the backward compatibility, etc. So it won't fix the issue that > > we can't use any bridge with any controller any time soon. > > I don't think that's the issue at hand here. You can still use a > non-OF-graph DT event if the nodes for the two bridges affected by this > patch define a port@0. It can just be left unconnected. > > I do agree it will cause some DT bindings for DCS-based DSI sinks to > have ports will others won't. If your concern is that all DT bindings > should be coherent, would you be OK with a patch that makes the sink > port mandatory in all DT bindings for DSI bridges and panels (and fixes > the mainline DT sources accordingly to make sure they validate) ? The > port would not be connected of course (at least when used with DSI > source drivers that don't use OF graph today). That would make DT > bindings coherent, and would be a first step towards using OF graph > everywhere. I'm trying to fix a (recent) mistake/cargo-cult in new bindings. That discussion is not going to be fairly controversial and I don't see how that can be solved quickly. So, as a second step, why not. But this one needs to come first. Maxime
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