On Friday, 14 September 2018 12:49:40 EEST Laurent Pinchart wrote: > Hi Stefan, > > On Thursday, 6 September 2018 23:25:56 EEST Stefan Agner wrote: > > On 06.09.2018 04:07, Linus Walleij wrote: > > > On Wed, Sep 5, 2018 at 8:32 PM Stefan Agner <stefan@xxxxxxxx> wrote: > > >> On 05.09.2018 00:44, Laurent Pinchart wrote: > > >> > > >> Good point! I actually really don't like that we use the same flags > > >> here > > >> but from a different perspective. Especially since the flags defines > > >> document things differently: > > >> > > >> /* drive data on pos. edge */ > > >> #define DRM_BUS_FLAG_PIXDATA_POSEDGE (1<<2) > > >> /* drive data on neg. edge */ > > >> #define DRM_BUS_FLAG_PIXDATA_NEGEDGE (1<<3) > > > > > > Maybe a stupid comment from my side, but can't we just change the > > > documentation to match the usecases? > > > > > > /* Trigger pixel data latch on positive edge */ > > > #define DRM_BUS_FLAG_PIXDATA_POSEDGE (1<<2) > > > > > >> Using the opposite perspective would also need translation in crtc > > >> drivers... So far no driver uses sampling_edge. > > >> > > >> I would prefer if we always use the meaning as documented by the flags. > > >> > > >> I guess we would need to convert DRM_BUS_FLAG_PIXDATA_POSEDGE -> > > >> DRM_BUS_FLAG_PIXDATA_NEGEDGE. > > >> > > >> Linus Walleij, you added sampling edge, any thoughts? > > > > > > I just thought it was generally useful to have triggering edge encoded > > > into the bridge as it makes it clear that this edge is something > > > that is a delayed version of the driving edge which is subject to > > > clock skew caused by the speed of electrons in silicon and > > > copper and slew rate caused by parasitic capacitance. > > > > Ok, I read a bit up on the history of bridge timing, especially: > > https://www.spinics.net/lists/dri-devel/msg155618.html > > > > IMHO, this got overengineered. For displays we do not need all that > > setup/sample delay timing information, and much longer cables are in > > use. So why is all that needed for bridges? > > > > For Linus case, the THS8134(A/B) data sheet I found (revised March 2010) > > clearly states: > > Clock input. A rising edge on CLK latches RPr0-7, GY0-7, BPb0-7. > > > > So we need to drive on negative edge, hence DRM_BUS_FLAG_PIXDATA_NEGEDGE > > should be used, which makes the pl111 driver setting TIM2_IPC: > > http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0121d/index > > .h tml > > > > > DRM_BUS_FLAG_PIXDATA_POSEDGE is the right value for my use cases, but it > > > doesn't match how the ADV7123 operates. Using > > > DRM_BUS_FLAG_PIXDATA_NEGEDGE > > > would match the hardware, but would break display for some modes, > > > depending on the display clock frequency as the internal 8.5ns output > > > delay applied to a falling clock edge would fall right into the 1.7ns > > > setup + hold time window of the ADV7123 around the rising edge. I can't > > > test this right now as I don't have local access to boards using the > > > ADV7123, but from a quick calculation that ignores the PCB transmission > > > delay modes with frequencies between 57MHz and 71MHz could break if the > > > data was output on the falling edge of the clock. > > > > If clocks vs. data signal are really that much off on R-Car DU, then > > parallel displays must have the very same issue... > > > > Are you sure that only the clock signal has an output delay? And that > > this output delay is a fixed value, clock independent? > > > > Typically, delays apply to all signals equally, and often are clock > > frequency dependent... > > > > Without really looking at the signals, I would not jump to conclusions > > here! I am pretty sure that driving on negative edge works just as well. > > I've tested Linus' original patch and it broke display on R-Car, so, no, it > doesn't work :-) > > The R-Car display engine delays the clock internally (in some cases that > delay is even configurable, and that's not uncommon in display > controllers). We really need all this information, and I believe we need it > for panels too, not just for bridges. The fact that we managed to get away > without adding it to panels is likely due to the large number of panels out > there, which makes it less likely that the same panel gets used by > different systems in mainline with different clock delays. I expect that > some panel drivers report the wrong clock edge to make things work on the > board they were tested with, and I expect we'll eventually need to add the > same information for panels too. > > So please don't remove this useful API, otherwise you'll break my board, and > I won't be happy. Or, to be precise, the board won't break now, but as soon as I need to implement support for configuring the output clock edge, it will break as the ADV7123 driver won't give me the right information to make the right choice. -- Regards, Laurent Pinchart
