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@xxxxxxxxxx> > >>>>> > >>>>> 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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