Re: [PATCH v6 1/8] drm/msm/dp: Add eDP support via aux_bus

From: Doug Anderson
Date: Sat Apr 02 2022 - 13:06:47 EST


On Sat, Apr 2, 2022 at 3:37 AM Dmitry Baryshkov
<dmitry.baryshkov@xxxxxxxxxx> wrote:
> On 01/04/2022 02:22, Doug Anderson wrote:
> > Hi,
> >
> > On Wed, Mar 30, 2022 at 9:03 AM Sankeerth Billakanti
> > <quic_sbillaka@xxxxxxxxxxx> wrote:
> >>
> >> @@ -1547,6 +1593,10 @@ int msm_dp_modeset_init(struct msm_dp *dp_display, struct drm_device *dev,
> >>
> >> dp_display->encoder = encoder;
> >>
> >> + ret = dp_display_get_next_bridge(dp_display);
> >> + if (ret)
> >> + return ret;
> >
> > It feels weird to me that this is in a function called "modeset_init",
> > though I certainly don't know the structure of the MSM display code
> > well enough to fully comment.
> It's called modeset_init() as it initializes KMS objects used by DP
> driver. We have similar functions for dsi and hdmi

Sorry, I wasn't meaning to imply that modeset_init() was a bad name or
anything. Mostly saying that I wasn't sure that modeset init was the
proper time to populate the aux bus. ...but then again, perhaps it is
given the current structure of this driver?

> > My expectation would have been that
> > devm_of_dp_aux_populate_ep_devices() would have been called from your
> > probe routine and then you would have returned -EPROBE_DEFER from your
> > probe if you were unable to find the panel afterwards.
> I don't think it's possible to call it from probe() since
> drm_dp_aux_register() is called only from dp_display_bind().
> The PHY also isn't initialized at that moment, so we can not probe AUX
> devices.
> The overall semantics of the AUX bus is not clear to me.
> Typically the bus is populated (and probed) when devices are accessible.
> But for the display related buses this might not be the case.

In general the AUX bus is modeled much like the i2c bus. You probe the
sub-device when you're able to transfer. Then you can confirm that the
device is actually there and init the device.

> For example for the DSI bus we clearly define that DSI transfer are not
> possible before the corresponding bridge's (or panel's) enable call.
> Maybe the same approach should be adopted for the AUX bus. This would
> allow us to populate the AUX bus before hardware access is actually
> possible, thus creating all the DRM bridges before the hardware is
> actually up and running.

So I guess what you're proposing is that you could probe the devices
under the AUX bus and they could acquire resources (and possibly
return EPROBE_DEFER) at a point in time _before_ it's actually
possible to transfer. Then I guess you'd later do the transfer?

I guess conceivably one could re-design the DRM subsystem like that,
but I don't think it's trivial. Why? I believe that we need to know
things about the panel at probe time. For instance, we need to be able
to populate the panel's modes. To get this information we need the
EDID which means we need to be able to do a transfer. If we're using
an AUX backlight we also need to add info about the backlight at probe
time and that also needs the transfer to work.

So I guess the net result is maybe we should just keep it where it is.
Long term I'd be interested in knowing if there's a reason why we
can't structure the driver so that AUX transfers can happen with less
intertwining with the rest of the code, but that can happen later. I
would expect that you'd basically just need clocks and regulators on
and maybe your PHY on. Ideally with some pm_runtime fun we should be
able to do that independently with anything else the driver needs to