Hi Laurent. On Tue, 22 Feb 2022 at 06:43, Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote: > > Hello, > > On Fri, Feb 18, 2022 at 02:20:19PM +0100, Andrzej Hajda wrote: > > On 16.02.2022 17:59, Dave Stevenson 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]. > > > > > > 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), > > API-wise that's my only concern, the flag should go somewhere else. Ah, the million dollar question then - where does it go? It is only true or false, so a bool in struct drm_bridge, same as interlace_allowed? I've had the realisation that this needs to be accessible from the panel drivers so DSI panel drivers such as panel-ilitek-ili9881 can set it too. I'll have a slight rethink over that one, but it is probably a similar extra flag in struct drm_panel. Dave > > > but does > > > this solve the problem posed? If not, then can you describe the actual scenario > > > it doesn't cover? > > > 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. > > > > Good summary, of multiple attempts of solving the issue (however I still > > could add some more :) ). > > Definitely good, thank you very much Dave for tackling this issue. > > > I think the main issue is that we try to squeeze different hardware > > protocol requirements into one quite restrictive framework - whole > > crtc->encoder->bridges->(panel ||connector) is managed directly by drm core. > > No place to negotiate configuration directly between players > > (bridges/panels). > > This patchset slightly looses the restrictions, so hopefully will help > > for some time, but still every developer needs to solve riddles what to > > put into callbacks, to allow driver working in different pipelines. > > That's true, but documentation can help a lot there. Patch 2/2 turns the > riddle-solving task into documentation reading. Granted, not everybody > will read the documentation (and we should probably link to it from the > documentation of the pre_enable and post_disable operations), but the > behaviour is now defined, which is a major step forward. > > > <DREAM MODE ON> > > Ideally I would like to drop idea of the bridge/panel and build > > abstraction on data links. > > So for example DSI/EDP bridge during probe would register DSI sink with > > their ops, and EDP source with their ops or just look for EDP sink (what > > will suit better). > > To establish data link they could use their ops and helpers to provide > > two-way conversation. > > This way if we need add support for new data link type or extend > > existing one we do not need to touch whole framework and pray to not > > break some strange bridge, or to add ops which will not be used by most > > of users. > > <DREAM MODE OFF> > > Protocol-specific operations can help, but I don't think they will > fundamentally change the problem. Yes, in some case, we can have > hardware requirements that are hard to express through generic > operations, but in most case the issue is more about defining the > semantic of the generic operations for a particular protocol than about > a need for a specific operation. > > The core issue, in my opinion, is that we have a mechanism that > essentially works from source to sink, with the source controlling the > sink. With some protocols (DSI in particular), the start sequence > requires more fine-grained control of the operations, and the sink > should be in control. We should ideally start a pipeline by calling the > enable operation on the last element (connector or panel), whose driver > will then call operations on its source, and interleave those calls with > control of the local device, in the exact sequence required by the > device. That's how the omapdrm driver operated before I ported it to > drm_bridge. Reversing the order of the pipeline enable was a huge piece > of work for a single driver. Doing it again in the other direction for > *all* drivers seems like an even bigger dream (or nightmare) than yours > Andrzej :-) > > > Putting dreams off, I think this patchset can add some value, at the > > price of call chain complication. Lets see opinion of others. > > I agree, I think it's a reasonable middleground. It improves the > situation, adds very little complexity in the API, has documentation to > specifies how the operations are meant to be implemented, and has a > reasonable increase of complexity for the pre_enable and post_disable > helpers (and the implementation could probably be simplified by moving > to recursive calls). I like this. > > > > [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(-) > > -- > Regards, > > Laurent Pinchart
