[PATCH V3 2/2] drm/bridge: ti-sn65dsi83: Add TI SN65DSI83 and SN65DSI84 driver

[Dave Stevenson]

On Mon, 10 May 2021 at 12:16, Marek Vasut <marex at denx.de> wrote:
>
> On 5/10/21 11:58 AM, Dave Stevenson wrote:
> > On Sat, 8 May 2021 at 21:26, Marek Vasut <marex at denx.de> wrote:
> >>
> >> On 5/7/21 2:48 PM, Dave Stevenson wrote:
> >>
> >> [...]
> >>
> >>>> +static void sn65dsi83_enable(struct drm_bridge *bridge)
> >>>> +{
> >>>> +       struct sn65dsi83 *ctx = bridge_to_sn65dsi83(bridge);
> >>>> +       unsigned int pval;
> >>>> +       u16 val;
> >>>> +       int ret;
> >>>> +
> >>>> +       /* Clear reset, disable PLL */
> >>>> +       regmap_write(ctx->regmap, REG_RC_RESET, 0x00);
> >>>> +       regmap_write(ctx->regmap, REG_RC_PLL_EN, 0x00);
> >>>
> >>> Sorry, a further thread of discussion coming from the investigations
> >>> I've been involved with.
> >>>
> >>> You've powered up in pre_enable, and are sending the I2C writes in enable.
> >>>
> >>> >From the docs for drm_bridge_funcs->enable[1]
> >>>
> >>>    * The bridge can assume that the display pipe (i.e. clocks and timing
> >>>    * signals) feeding it is running when this callback is called. This
> >>>    * callback must enable the display link feeding the next bridge in the
> >>>    * chain if there is one.
> >>>
> >>> So video is running when enable is called, and the DSI data lanes may
> >>> be HS. (Someone correct me if that is an incorrect reading of the
> >>> text).
> >>>
> >>> The SN65DSI84 datasheet table 7-2 Initialization Sequence gives init
> >>> seq 8 as being "Change DSI data lanes to HS state and start DSI video
> >>> stream", AFTER all the I2C has been completed except reading back
> >>> registers and checking for errors.
> >>> With video running you don't fulfil the second part of init seq 2 "the
> >>> DSI data lanes MUST be driven to LP11 state"
> >>>
> >>> My investigations have been over delaying starting the DSI video
> >>> stream until after enable, but reading the descriptive text for enable
> >>> I believe the Pi is correct to be sending video at that point.
> >>> I guess there is some ambiguity as to whether the clock lane is going
> >>> to be in HS mode during pre_enable. On the Pi the PHY and clocks will
> >>> be enabled prior to pre_enable to allow for sending DSI commands
> >>> during pre_enable, but it may not be true on other platforms.
> >>
> >> You have to make sure the clock lane is running and in HS mode when
> >> configuring the DSI83, otherwise the internal DSI83 state machine won't
> >> be able to operate.
> >
> > Indeed, but my reading of the documentation says that neither
> > pre_enable nor enable give you the state that you require.
> > You need a hook in the middle, an option to ask for clock lanes during
> > pre_enable or no video during enable, or an amendment to the docs over
> > the state during enable.
> >
> > Having the data lanes in HS mode does appear to stop the DSI83
> > accepting the I2C setup commands.
>
> Uhh, that is new. Is that what you observed in your lab ?
>
> I saw the DSI83 behave this way if the clock lane was stopped, but the
> data lanes had no impact. Was your clock lane running when the DSI83 was
> not accepting i2c commands ? Does your DSI83 source clock from it or
> from external Xtal ?

I haven't got into the lab as yet, and I don't have a DSI83 myself.
This is relaying experimentation from others.
They're using the DSI clock lane as the clock source.Yes the clock
lane on the Pi is started before any of the enable bridge calls.

In the vc4 driver[1] it runs through the all pre-enables, configures
register DISP0_CTRL including setting bit DSI_DISP0_ENABLE which
starts it requesting pixels from the pipeline, and then calls all the
enables. With that behaviour it fails to start the DSI83.

If the DSI83 I2C setup code is moved from enable to pre_enable then it
works, or if patch [2] is used to move the setting of the
DSI_DISP0_ENABLE bit to after enable it also works.

Sorry life is all rather up in the air with working from home. I'll go
into the lab and try to confirm that DSI_DISP0_ENABLE does what the
documentation implies it does.
Those who do have hardware now have it working on the Pi, although
with a version of Jagan's driver rather than yours. We're trying to
figure out the diffs with yours.

If you have it working reliably on other platforms that you believe
are following the docs during pre_enable and enable, then I'm happy to
drop out of the discussions for now. We can revisit it once we have
determined exactly why it's being fussy on the Pi.

Cheers
  Dave

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/vc4/vc4_dsi.c#L1072
[2] https://github.com/6by9/linux/commit/b939eaffc47cc84ebfea6bf1ab10ae1ec9fa58c2