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. I can also forward this internally, but I expect it to have more weight coming directly from Cisco. =) > Apologies for the confusion, I should never send emails after 5pm :-) No worries. Thierry
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