Re: [PATCH 1/2] media: tegra-cec: Support Tegra186 and Tegra194

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Dec 11, 2018 at 10:40:55AM +0100, Hans Verkuil wrote:
> (Resend, this time with a proper reply)
> 
> On 12/11/18 10:38 AM, Thierry Reding wrote:
> > On Tue, Dec 11, 2018 at 10:19:48AM +0100, Hans Verkuil wrote:
> >> On 12/10/18 9:59 PM, Thierry Reding wrote:
> >>> On Mon, Dec 10, 2018 at 06:07:10PM +0100, Hans Verkuil wrote:
> >>>> Hi Thierry,
> >>>>
> >>>> On 12/10/18 5:00 PM, Thierry Reding wrote:
> >>>>> From: Thierry Reding <treding@nvidia.com>
> >>>>>
> >>>>> The CEC controller found on Tegra186 and Tegra194 is the same as on
> >>>>> earlier generations.
> >>>>
> >>>> Well... at least for the Tegra186 there is a problem that needs to be addressed first.
> >>>> No idea if this was solved for the Tegra194, it might be present there as well.
> >>>>
> >>>> The Tegra186 hardware connected the CEC lines of both HDMI outputs together. This is
> >>>> a HW bug, and it means that only one of the two HDMI outputs can use the CEC block.
> >>>
> >>> I don't know where you got that information from, but I can't find any
> >>> indication of that in the documentation. My understanding is that there
> >>> is a single CEC block that is completely independent and it is merely a
> >>> decision of the board designer where to connect it. I'm not aware of any
> >>> boards that expose more than a single CEC.
> >>
> >> Sorry, my memory was not completely correct.
> >>
> >> The problem is that the 186 can be configured with two HDMI outputs, but it has
> >> only one CEC block. So CEC can be used for only one of the two. I checked the TRM
> >> for the Tegra194 and that has up to four HDMI outputs, but still only one CEC
> >> block.
> >>
> >> And yes, it is the responsibility for the board designer to hook up the CEC pin
> >> to only one of the outputs, but the TRM never explicitly mentions this and given
> >> the general lack of knowledge about CEC it wouldn't surprise me at all if there
> >> will be wrong board designs.
> >>
> >> But be that as it may, the core problem remains: you cannot allow multiple
> >> HDMI outputs to be connected to the same CEC device.
> >>
> >> However, I now realize that your patches will actually work fine since each
> >> HDMI connector tries to get a cec notifier for its own HDMI device, but the
> >> tegra-cec driver will only register a notifier for the HDMI device pointed
> >> to by the hdmi-phandle property. So only one of the HDMI devices will actually
> >> get a working CEC.
> >>
> >> Although if board designers mess this up and connect multiple CEC lines to
> >> the same CEC pin, this would still break, but there is nothing that can be
> >> done about that. I still believe the TRM should have made this clear since
> >> it is not obvious. Even better would be to have the same number of CEC blocks
> >> as there are configurable HDMI outputs. Typically, if you support CEC on one
> >> HDMI output, you want to support it for all. And today that's not possible
> >> without adding external CEC devices (as we - Cisco - do).
> > 
> > I wasn't aware that anyone was using a Tegra with support for multiple
> > HDMI outputs. Do you have a contact that you can forward this kind of
> > request to? It certainly sounds like something that would be useful to
> > add in future chips if there's a customer need.
> 
> We have contacts, and we did report it, but nothing happened with it
> AFAIK. It was likely too late for changes to the Tegra194 design in any
> case.

Okay. I'll follow up internally, see if I can find out what the story
is.

Thanks,
Thierry

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