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

[Michael Walle]

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@xxxxxxxxxx>
> > ---
> > 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