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

[Heikki Krogerus]

Hi guys,

On Wed, Oct 06, 2021 at 01:26:35PM -0700, Prashant Malani wrote:
> (CC+ Heikki)
> 
> Hi,
> 
> On Wed, Oct 6, 2021 at 8:19 AM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On Tue, Oct 5, 2021 at 7:27 PM Bjorn Andersson
> > <bjorn.andersson@xxxxxxxxxx> 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@xxxxxxxxxx/
> > > >
> > > > 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?

I didn't go over the whole thread, so I may have misunderstood
something, but I don't think this has anything to do with muxes. The
mux should not be a problem for the DRM side under no circumstance.
Like Doug said, the mux API is being abused here.

HPD was one use case here, so I'll try to explain how that happens...

If the USB Type-C connector is in DP alt mode, then ideally your USB
Type-C controller/port driver has registered the partner device DP alt
mode the moment it detected that the partner supports that mode, and
that partner DP alt mode will have then been bind to the DP alt mode
driver:

        drivers/usb/typec/altmodes/displayport.c

After that, if the DP alt mode driver sees HPD - HPD is message
signalled in DP alt mode (in case some of you guys didn't know) - the
DP alt mode driver notifies the DRM connector about it by calling
this function:

        void drm_connector_oob_hotplug_event(struct fwnode_handle *connector_fwnode);

If your USB Type-C controller/port driver does not yet register the DP
alt mode, the it's responsible of handling HPD separately by calling
drm_connector_oob_hotplug_event() on its own.

Either way, the only thing needed here is description of the
connection between the USB Type-C connector and the DisplayPort in
firmware - the mux is not relevant here. There are no DT bindings
defined for that AFAIK (or are there?), but presumable you want to use
OF graph with DT. Right now the DP alt mode driver does not try to
find the connection from device graph (so OF graph), but it should not
be a problem to add support for it.


> > > > ...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

thanks,

-- 
heikki