On Tue, Jan 10, 2017 at 05:33:15PM +0200, Ville Syrjälä wrote: > On Tue, Jan 10, 2017 at 12:26:53PM +0000, Jose Abreu wrote: > > Hi Ville, > > > > > > On 10-01-2017 11:16, Ville Syrjälä wrote: > > > On Thu, Jan 05, 2017 at 02:46:06PM +0000, Jose Abreu wrote: > > >> > > > > [snip] > > > > >> The pixel clock rate is half of the TMDS character rate in 4:2:0 > > >> (in 24 bit), but for example in deep color 48 bit it will be the > > >> same rate. There is also a reduction to half of htotal, hactive, > > >> hblank, hfront, hsync and hback but I don't think it's a good > > >> solution to guide us from there. > > > I was asking if we can look at a specific modeline and whether we > > > can tell from that if we would need to output it as 4:2:0. > > > > Hmm, according to HDMI 2.0 spec there are no 4:2:0 only modes and > > the only way to figure out if the mode is 4:2:0 only (or able) is > > to parse the VCB and VBD blocks from EDID. The clock is half rate > > but this is the source that has to figure it out. The mode is > > still passed in a regular way (By VIC, by timing, ...). > > > > > > > >> Why does it feel wrong to you > > >> expanding the uapi? > > > Because it requires changing every single userspace kms client. And > > > it's not something userspace should have to worry about. > > > > I agree but, as Daniel said [1], we could make these new HDMI 2.0 > > features optional and only pass them to userspace if client asked > > for them. What do you think? > > Are you going to update all the userspace clients? Exposing HDMI 2.0 > modes only for your favorite client doesn't sound like a good plan to > me. > > If we simply compute from a specific modeline whether it needs to be > transmitted as 4:2:0, I suppose we could simply look for a matching > mode in the 4:2:0 mode. But that would mean that only the exact modes > listed by the EDID will work, and others might not. OK, so the 4:2:0 support is apparently listed only for specific VICs. Hence we will need to just go through those lists to see if we can (or in fact must) use 4:2:0 for a specific user specified mode. I would say the only slight question mark at this point is whether we should favor 4:4:4 at lower bpc or 4:2:0 at higher bpc if we can choose between the two. My first instinct is to favor the 4:4:4 modes for now. We could add some knobs later to let the user make that choice. > > > > > [1] > > https://lists.freedesktop.org/archives/dri-devel/2017-January/128683.html > > > > > > > >> I think its important to say that the chosen colorspace can > > >> improve performance in systems: for example, as I said, 4:2:0 > > >> 24-bit uses half the rate that RGB 24-bit uses so we have less > > >> trafic in the bus. I am recently working with a FPGA connected > > >> trough pcie and I can definitely say that this is true. But, as > > >> expected, less traffic means less quality in final image so its > > >> not a matter of letting kernel decide, I think its a matter of > > >> user choosing between performance vs. quality. > > > Image quality control for userspace is a much bigger topic. And > > > something we have no real precedent for at the moment (apart from > > > user choosing a different fb pixel format). > > > > > > The performance arument is very hardware dependent, and not really > > > all that relevant IMO. If the user wants the big mode they either > > > get it or not depending on whether the system can deliver. > > > > > > > Ok. But note that there is no nice way to figure this out. For > > example, for a graphics card it all depends (apart from the > > graphics HW) on the PCIe bus. If the bus is not free for enough > > data rate then user can reach bottlenecks and not output at best > > performance. If we gave user the ability to switch from, for > > example, RGB to YCbCr 4:2:0 this bottleneck could be eliminated. > > Userspace won't know anything about such bottlenecks. The kernel > can know it and hence should automagically drop into 4:2:0 mode > if necessary. > > > Unless of course we always prefer YCbCr 4:2:0, when possible. I > > did this internally for bridge driver dw-hdmi. We always prefer > > YCbCr over RGB when they are available. It is user transparent as > > the controller does the necessary color space conversion, though, > > not ideal in my opinion. > > My idea was that we'd have a property for the output colorspace and > would perhaps default to YCbCr for the CEA modes (as per CEA-861). > Though I'm sure some people would cry about that behaviour as well. > But for the cases where there is no choice but to use a specific > output colorspace, the kernel should just do it automagically IMO. No > point in manking life diffcult for userspace. > > -- > Ville Syrjälä > Intel OTC -- Ville Syrjälä Intel OTC _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
