On Wed, 16 Feb 2022 at 20:00, 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]. I have been thinking about a similar approach: - Add power_up() and power_down() ops to drm_bridge_ops - Add separate drm_bridge_chain_power_up() and _power_down() with power_up being called from encoder to the connector end of the chain and power_down being called in the opposite direction. - Make those functions call power_up()/power_down() if it's available and call pre_enable()/post_disable() if it is not - Make drm_bridge_chain_pre_enable()/_post_disable() call pre_enable()/post_disable() only if power_up()/power_down() are not present. This removes the immediate need for the rework of the drivers. On the other hand the DSI hosts can be brought up in the power_up stage (to the LP-11). Then bridge's pre_enable() can communicate with the actual hardware in time. So does bridge's post_disable() > This patch takes a hybrid of the two: an optional reversing of the order for > specific links within the bridge chain within pre_enable and post_disable done > within the drm_bridge framework. > I'm more than happy to move where the flag exists in structures (currently as > DRM_BRIDGE_OP_UPSTREAM_FIRST in drm_bridge_ops, but it isn't an op), but does > this solve the problem posed? If not, then can you describe the actual scenario > it doesn't cover? My general feeling is that this makes the chain traversing functions unnecessarily complex. > A DSI peripheral can set the flag to get the DSI host initialised first, and > therefore it has a stable LP-11 state before pre_enable. Likewise the peripheral > can still send shutdown commands prior to the DSI host being shut down in > post_disable. It also handles the case where there are multiple devices in the > chain that all want their upstream bridge enabled first, so should there be a > DSI mux between host and peripheral, then it can still get the host to the > correct state. > > An example tree is at [7] which is drm-misc-next with these patches and then a > conversion of vc4_dsi to use the atomic bridge functions (will be upstreamed > once we're over this hurdle). It is working happily with the Toshiba TC358762 on > a Raspberry Pi 7" panel. > The same approach but on our vendor 5.15 tree[8] has also been tested > successfully on a TI SN65DSI83 and LVDS panel. > > Whilst here, I've also documented the expected behaviour of DSI hosts and > peripherals to aid those who come along after. > > Thanks > Dave > > [1] https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c#L940 > [2] https://lists.freedesktop.org/archives/dri-devel/2022-January/337769.html > [3] https://lists.freedesktop.org/archives/dri-devel/2021-December/333908.html > [4] https://lists.freedesktop.org/archives/dri-devel/2021-October/328476.html > [5] https://lists.freedesktop.org/archives/dri-devel/2021-October/325853.html > [6] https://lists.freedesktop.org/archives/dri-devel/2022-February/341852.html > [7] https://github.com/6by9/linux/tree/drm-misc-next-vc4_dsi > [8] https://github.com/6by9/linux/tree/rpi-5.15.y-sn65dsi83 > > > Dave Stevenson (2): > drm: Introduce DRM_BRIDGE_OP_UPSTREAM_FIRST to alter bridge init order > drm/bridge: Document the expected behaviour of DSI host controllers > > Documentation/gpu/drm-kms-helpers.rst | 7 + > drivers/gpu/drm/drm_bridge.c | 235 ++++++++++++++++++++++++++++++---- > include/drm/drm_bridge.h | 8 ++ > 3 files changed, 225 insertions(+), 25 deletions(-) > > -- > 2.7.4 > -- With best wishes Dmitry
