Hi, On Wed, Feb 16, 2022 at 9:00 AM Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx> wrote: > > Hi All > > Hopefully I've cc'ed all those that have bashed this problem around previously, > or are otherwise linked to DRM bridges. > > There have been numerous discussions around how DSI support is currently broken > as it doesn't support initialising the PHY to LP-11 and potentially the clock > lane to HS prior to configuring the DSI peripheral. There is no op where the > interface is initialised but HS video isn't also being sent. > Currently you have: > - peripheral pre_enable (host not initialised yet) > - host pre_enable > - encoder enable > - host enable > - peripheral enable (video already running) > > vc4 and exynos currently implement the DSI host as an encoder, and split the > bridge_chain. This fails if you want to switch to being a bridge and/or use > atomic calls as the state of all the elements split off are not added by > drm_atomic_add_encoder_bridges. > > dw-mipi-dsi[1] and now msm[2] use the mode_set hook to initialise the PHY, so > the bridge/panel pre_enable can send commands. In their post_disable they then > call the downstream bridge/panel post_disable op manually so that shutdown > commands can be sent before shutting down the PHY. Nothing handles that fact, > so the framework then continues down the bridge chain and calls the post_disable > again, so we get unbalanced panel prepare/unprepare calls being reported [3]. > > There have been patches[4] proposing reversing the entire direction of > pre_enable and post_disable, but that risks driving voltage into devices that > have yet to be powered up. > There have been discussions about adding either a pre_pre_enable, or adding a > DSI host_op to initialise the host[5]. Both require significant reworking to all > existing drivers in moving initialisation phases. > We have patches that look like they may well be addressing race conditions in > starting up a DSI peripheral[6]. In general I'm happy to let the more senior people in DRM set the direction here so I probably won't do lots of review, but I will point out that I did have another proposal that sorta got lost in the noise of the whole "reversing the entire direction". That's basically: https://lists.freedesktop.org/archives/dri-devel/2021-October/328934.html I have no idea if something like that would work for your use case, but after analyzing it it felt like a surprisingly clean proposal even if my first instinct when I thought about it was that it was a hack. ;-) I suspect (but haven't analyzed your code) that it might be equivalent to your proposal of using a flag but maybe easier to wrap ones head around? -Doug
