Re: [RFC] drm: Parse HDMI 2.0 YCbCr 4:2:0 VDB and VCB

[Ville Syrjälä]

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.

> 
> [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