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

From: Doug Anderson
Date: Tue Apr 05 2022 - 16:15:56 EST


On Tue, Apr 5, 2022 at 10:36 AM Dmitry Baryshkov
<dmitry.baryshkov@xxxxxxxxxx> wrote:
> On 05/04/2022 20:02, Doug Anderson wrote:
> > Hi,
> >
> > On Tue, Apr 5, 2022 at 5:54 AM Dmitry Baryshkov
> > <dmitry.baryshkov@xxxxxxxxxx> wrote:
> >>> 3. For DP and eDP HPD means something a little different. Essentially
> >>> there are two concepts: a) is a display physically connected and b) is
> >>> the display powered up and ready. For DP, the two are really tied
> >>> together. From the kernel's point of view you never "power down" a DP
> >>> display and you can't detect that it's physically connected until it's
> >>> ready. Said another way, on you tie "is a display there" to the HPD
> >>> line and the moment a display is there it's ready for you to do AUX
> >>> transfers. For eDP, in the lowest power state of a display it _won't_
> >>> assert its "HPD" signal. However, it's still physically present. For
> >>> eDP you simply have to _assume_ it's present without any actual proof
> >>> since you can't get proof until you power it up. Thus for eDP, you
> >>> report that the display is there as soon as we're asked. We can't
> >>> _talk_ to the display yet, though. So in get_modes() we need to be
> >>> able to power the display on enough to talk over the AUX channel to
> >>> it. As part of this, we wait for the signal named "HPD" which really
> >>> means "panel finished powering on" in this context.
> >>>
> >>> NOTE: for aux transfer, we don't have the _display_ pipe and clocks
> >>> running. We only have enough stuff running to do the AUX transfer.
> >>> We're not clocking out pixels. We haven't fully powered on the
> >>> display. The AUX transfer is designed to be something that can be done
> >>> early _before_ you turn on the display.
> >>>
> >>>
> >>> OK, so basically that was a longwinded way of saying: yes, we could
> >>> avoid the AUX transfer in probe, but we can't wait all the way to
> >>> enable. We have to be able to transfer in get_modes(). If you think
> >>> that's helpful I think it'd be a pretty easy patch to write even if it
> >>> would look a tad bit awkward IMO. Let me know if you want me to post
> >>> it up.
> >>
> >> I think it would be a good idea. At least it will allow us to judge,
> >> which is the more correct way.
> >
> > I'm still happy to prototype this, but the more I think about it the
> > more it feels like a workaround for the Qualcomm driver. The eDP panel
> > driver is actually given a pointer to the AUX bus at probe time. It's
> > really weird to say that we can't do a transfer on it yet... As you
> > said, this is a little sideband bus. It should be able to be used
> > without all the full blown infra of the rest of the driver.
> Yes, I have that feeling too. However I also have a feeling that just
> powering up the PHY before the bus probe is ... a hack. There are no
> obvious stopgaps for the driver not to power it down later.

This is why I think we need to move to Runtime PM to manage this. Basically:

1. When an AUX transfer happens, you grab a PM runtime reference that
_that_ powers up the PHY.

2. At the end of the AUX transfer function, you do a "put_autosuspend".

Then it becomes not a hack, right?

> A cleaner design might be to split all hotplug event handling from the
> dp_display, provide a lightweight state machine for the eDP and select
> which state machine to use depending on the hardware connector type. The
> dp_display_bind/unbind would probably also be duplicated and receive
> correct code flows for calling dp_parser_get_next_bridge, etc.
> Basically that means that depending on the device data we'd use either
> dp_display_comp_ops or (new) edp_comp_ops.

I don't think I know the structure of the MSM DP code to make a
definitive answer here. I think I'd have to see a patch. However I'd
agree in general terms that we need some different flows for the two.
;-) We definitely want to limit the differences but some of them will
be unavoidable...