[PATCH 0/2] DSI host and peripheral initialisation ordering

[Dave Stevenson]

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), 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.

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(-)

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2.7.4