Re: [PATCH] drm: bridge: samsung-dsim: Don't use FORCE_STOP_STATE

[Michael Walle <mwalle@kernel.org>]

Hi Alexander,

>> 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.
> 
> Apparently LP-11 is actually entered with the call to
> samsung_dsim_enable_lane(), but I don't know about other requisites on 
> that
> matter. Unfortunately documentation lacks a lot in that regard.

Which is called during samsung_dsim_atomic_pre_enable(). So I'm not sure
why the FORCE_STOP_STATE was needed at all. Lanes will be in LP-11 mode
if the video stream isn't enabled.

>> 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.
> 
> At which point does SN65DSI84 require LP-11?

I guess I have the same requirement as Frieder (we use the same bridge).
According to the datasheet, the DSI data must be in LP-11 when releasing
EN. According to the init sequence:
  - asserting EN
  - configure CSRs
  - enable video stream

Although not, stated explicitly, LP-11 should also be active during CSR
writes.

But after all, the DSIM driver should adhere to the linux requirement,
which Frieder cited [1].

> You have access to a DSI/D-PHY analyzer?

A pretty fast oscilloscope with an DSI decoder. So we can analyze one 
DSI
lane.

>> 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.
> 
> AFAIK ti-sn65dsi83.c only uses I2C for communication. Did you send DSI 
> read/
> writes instead?

Yeah the bridge only supports I2C. I just wanted to try that a DSI 
command
will still work after this patch. As a quick hack, I just added an
mipi_dsi_generic_write() to sn65dsi83_atomic_pre_enable() and made sure
there is a DSI write packet on the actual link.

-michael

> 
> best regards,
> Alexander
> 
>> 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@kernel.org>
>> ---
>> Let me know wether this should be two commits each reverting one, but 
>> both
>> commits appeared first in kernel 6.5.
>> 

[1] 
https://docs.kernel.org/gpu/drm-kms-helpers.html#mipi-dsi-bridge-operation