The FORCE_STOP_STATE bit is unsuitable to force the DSI link into LP-11
mode. It seems the bridge internally queues DSI packets and when the
FORCE_STOP_STATE bit is cleared, they are sent in close succession
without any useful timing (this also means that the DSI lanes won't go
into LP-11 mode). The length of this gibberish varies between 1ms and
5ms. This sometimes breaks an attached bridge (TI SN65DSI84 in this
case). In our case, the bridge will fail in about 1 per 500 reboots.
The FORCE_STOP_STATE handling was introduced to have the DSI lanes in
LP-11 state during the .pre_enable phase. But as it turns out, none of
this is needed at all. Between samsung_dsim_init() and
samsung_dsim_set_display_enable() the lanes are already in LP-11 mode.
The code as it was before commit 20c827683de0 ("drm: bridge:
samsung-dsim: Fix init during host transfer") and 0c14d3130654 ("drm:
bridge: samsung-dsim: Fix i.MX8M enable flow to meet spec") was correct
in this regard.
This patch basically reverts both commits. It was tested on an i.MX8M
SoC with an SN65DSI84 bridge. The signals were probed and the DSI
packets were decoded during initialization and link start-up. After
this
patch the first DSI packet on the link is a VSYNC packet and the timing
is correct.
Command mode between .pre_enable and .enable was also briefly tested by
a quick hack. There was no DSI link partner which would have responded,
but it was made sure the DSI packet was send on the link. As a side
note, the command mode seems to just work in HS mode. I couldn't find
that the bridge will handle commands in LP mode.
Fixes: 20c827683de0 ("drm: bridge: samsung-dsim: Fix init during host
transfer")
Fixes: 0c14d3130654 ("drm: bridge: samsung-dsim: Fix i.MX8M enable flow
to meet spec")
Signed-off-by: Michael Walle <mwalle@xxxxxxxxxx>
---
Let me know wether this should be two commits each reverting one, but
both
commits appeared first in kernel 6.5.
Are there any news?
-michael
