Re: [RFC] drm/msm/dp: Allow attaching a drm_panel

[Prashant Malani <pmalani@chromium.org>]

(CC+ Heikki)

Hi,

On Wed, Oct 6, 2021 at 8:19 AM Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Tue, Oct 5, 2021 at 7:27 PM Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> >
> > > > For reference, this is how I thought one is supposed to tie the Type-C
> > > > controller to the display driver:
> > > > https://lore.kernel.org/all/20211005022451.2037405-1-bjorn.andersson@linaro.org/
> > >
> > > OK, so I looked at that a bit. Fair warning that I've never looked at
> > > the type C code before today so anything I say could be totally wrong!
> > > :-)
> > >
> > > ...but I _think_ you're abusing the "mux" API for this. I think a type
> > > C port can have exactly 1 mux, right? Right now you are claiming to be
> > > _the_ mux in the DP driver, but what about for other alt modes? If
> > > those wanted to be notified about similar things it would be
> > > impossible because you're already _the_ mux, right?
> > >
> >
> > I actually don't think so, because I acquire the typec_mux handle by the
> > means of:
> >
> > mux_desc.svid = USB_TYPEC_DP_SID;
> > mux_desc.mode = USB_TYPEC_DP_MODE;
> > alt_port->mux = fwnode_typec_mux_get(fwnode, &mux_desc);
>
> Hrm, I guess I need to go find that code. Ah, I see it in your WIP
> tree, but not posted anywhere. :-P The only code I can see calling
> fwnode_typec_mux_get() is `drivers/platform/chrome/cros_ec_typec.c`.
> In that code it passes NULL for the mux_desc and I'm nearly certain
> that it just handles one "mux" per connector despite the fact that it
> handles lots of different types of alternate modes. That doesn't mean
> that the cros_ec implementation is correct / finalized, but it's a
> reference point.
>
>
> > And in the DisplayPort node I provide svid = /bits/ 16 <0xff01>;
> >
> > So I will be able to reference multiple different altmode
> > implementors using this scheme.
>
> OK, so I'm trying to grok this more. Let's see.
>
> I'm looking at ucsi_glink_probe() and looking at the matching dts in
> your WIP tree [1] in "sc8180x-lenovo-flex-5g.dts" OK, so:
>
> 1. It's looping once per _connector_ by looping with
> `device_for_each_child_node(dev, fwnode)`.
>
> 2. For each connector, it has exactly one `alt_port` structure.
>
> 3. For each `alt_port` structure it has exactly one `mux`.
>
> ...so currently with your WIP tree there is one "mux" per type C connector.
>
>
> Perhaps what you're saying, though, is that the UCSI code in your WIP
> tree can/should be changed to support more than one mux per port. Then
> I guess it would have logic figuring out what muxes to notify about
> which things? ...and I guess that would mean that it's currently a bug
> that the ucsi_altmode_enable_usb() notifies "the DP type C mux" about
> USB changes?
>
>
> > > I _think_ a mux is supposed to be something more like
> > > `drivers/phy/rockchip/phy-rockchip-typec.c` (though that code predates
> > > the type C framework we're looking at here). There the phy can do all
> > > the work of remuxing things / flipping orientation / etc. I don't
> > > think it's a requirement that every SoC be able to do this remuxing
> > > itself but (if memory serves) rk3399 implemented it so we didn't have
> > > to do it on the TCPC and could use a cheaper solution there.
> > >
> >
> > I'm afraid I don't see how this interacts with a display controller.
>
> This was actually kinda my point. ;-) Specifically I think
> `phy-rockchip-typec.c` is the thing that's supposed to be a "mux". I
> think your display controller isn't a mux. Yeah, it's handy that muxes
> get told about DP HPD status, but that doesn't mean it's the right
> abstraction for you to implement. In my mental model, it's the same as
> implementing your "i2c" controller with a "pinctrl" driver. :-P
>
>
> > It
> > seems more like it's the phy side of things, what we have split between
> > the Type-C controller and the QMP phy to set the pins in the right
> > state.
> >
> > > In any case, my point is that I think there is supposed to be a
> > > _single_ mux per port that handles reassigning pins and that's what
> > > this API is for.
> > >
> >
> > If that's the case things such as typec_mux_match() is just completely
> > backwards.
>
> Yeah, I have no explanation for typec_mux_match(). Let me see if I can
> lure some type C folks into this discussion.

This aligns with the model I have in my mind (not that that is
necessarily the right one).
I took that matching code to be meant to handle cases where the
firmware doesn't explicitly
define a "mode-switch" for the port (and so we look at the SVIDs
listed in the Mux fwnode descriptor).

The matcher code does suggest there could be a mux for each alternate
mode. But then, how does the
bus code know which mux to set [2] ? In that code, the struct altmode
has a pointer to the struct typec_mux, but I
don't see where that pointer is assigned. I assumed that it was set to
whatever the mux node of the
Type C port was whenever the port driver registered its altmodes for
each port, but I can't substantiate
that assumption in code.

Heikki, do you have any guidance regarding what the expected usage is
here? One typec_mux struct per type C port? Or
1 typec_mux per altmode per port?

>
>
> > > ...so I will still assert that the right thing to do is to have a
> > > drm_bridge for the type c connector and _that's_ what should be
> > > sending HPD.
> > >
> >
> > That still implies that all the current typec_mux code got it all wrong
> > and should be thrown out. If you instead consider that you have a Type-C
> > controller that upon switching DisplayPort on/off calls typec_mux_set()
> > to inform the functions that things has changed then all the current
> > code makes sense.
> >
> > It also maps nicely to how the TypeC controller would call
> > typec_switch_set() to inform, in our case the QMP phy that the
> > orientation has switched.
> >
> >
> > It seems reasonable to have some common helper code that registers the
> > typec_mux and turn its notifications into HPD notifications to the
> > display code, but I still think that should live in the DRM framework,
> > separate from the USB code.
>
> I think I'm going to step back and hope that the experts can chime in.
>
>
> [1] https://github.com/andersson/kernel/commits/wip/sc8180x-next-20210819
[2]: https://elixir.bootlin.com/linux/v5.15-rc4/source/drivers/usb/typec/bus.c#L27

>
> -Doug