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. > > 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 >
