Subject: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

From: [email-masked] (Tom Cooksey)
Date: Mon, 5 Aug 2013 18:10:11 +0100

Hi Rob,

+linux-media, +linaro-mm-sig for discussion of video/camera
buffer constraints...

> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <[email-masked]>
> wrote:
> >> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
> >> >    allocate buffers for the GPU. Still not sure how to resolve
> >> >    this as we don't use DRM for our GPU driver.
> >>
> >> any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
> >> once the dma-fence stuff is in place), we'd have gpu-specific drm
> >> (gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
> >> using prime/dmabuf to share between the two.
> >
> > The "extra" buffers we were allocating from armsoc DDX were really
> > being allocated through DRM/GEM so we could get an flink name
> > for them and pass a reference to them back to our GPU driver on
> > the client side. If it weren't for our need to access those
> > extra off-screen buffers with the GPU we wouldn't need to
> > allocate them with DRM at all. So, given they are really "GPU"
> > buffers, it does absolutely make sense to allocate them in a
> > different driver to the display driver.
> >
> > However, to avoid unnecessary memcpys & related cache
> > maintenance ops, we'd also like the GPU to render into buffers
> > which are scanned out by the display controller. So let's say
> > we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
> > out buffers with the display's DRM driver but a custom ioctl
> > on the GPU's DRM driver to allocate non scanout, off-screen
> > buffers. Sounds great, but I don't think that really works
> > with DRI2. If we used two drivers to allocate buffers, which
> > of those drivers do we return in DRI2ConnectReply? Even if we
> > solve that somehow, GEM flink names are name-spaced to a
> > single device node (AFAIK). So when we do a DRI2GetBuffers,
> > how does the EGL in the client know which DRM device owns GEM
> > flink name "1234"? We'd need some pretty dirty hacks.
> 
> You would return the name of the display driver allocating the
> buffers.  On the client side you can use generic ioctls to go from
> flink -> handle -> dmabuf.  So the client side would end up opening
> both the display drm device and the gpu, but without needing to know
> too much about the display.

I think the bit I was missing was that a GEM bo for a buffer imported
using dma_buf/PRIME can still be flink'd. So the display controller's
DRM driver allocates scan-out buffers via the DUMB buffer allocate
ioctl. Those scan-out buffers than then be exported from the
dispaly's DRM driver and imported into the GPU's DRM driver using
PRIME. Once imported into the GPU's driver, we can use flink to get a
name for that buffer within the GPU DRM driver's name-space to return
to the DRI2 client. That same namespace is also what DRI2 back-buffers
are allocated from, so I think that could work... Except...

> > Anyway, that latter case also gets quite difficult. The "GPU"
> > DRM driver would need to know the constraints of the display
> > controller when allocating buffers intended to be scanned out.
> > For example, pl111 typically isn't behind an IOMMU and so
> > requires physically contiguous memory. We'd have to teach the
> > GPU's DRM driver about the constraints of the display HW. Not
> > exactly a clean driver model. :-(
> >
> > I'm still a little stuck on how to proceed, so any ideas
> > would greatly appreciated! My current train of thought is
> > having a kind of SoC-specific DRM driver which allocates
> > buffers for both display and GPU within a single GEM
> > namespace. That SoC-specific DRM driver could then know the
> > constraints of both the GPU and the display HW. We could then
> > use PRIME to export buffers allocated with the SoC DRM driver
> > and import them into the GPU and/or display DRM driver.
> 
> Usually if the display drm driver is allocating the buffers that might
> be scanned out, it just needs to have minimal knowledge of the GPU
> (pitch alignment constraints).  I don't think we need a 3rd device
> just to allocate buffers.

While Mali can render to pretty much any buffer, there is a mild
performance improvement to be had if the buffer stride is aligned to
the AXI bus's max burst length when drawing to the buffer.

So in some respects, there is a constraint on how buffers which will
be drawn to using the GPU are allocated. I don't really like the idea
of teaching the display controller DRM driver about the GPU buffer
constraints, even if they are fairly trivial like this. If the same
display HW IP is being used on several SoCs, it seems wrong somehow
to enforce those GPU constraints if some of those SoCs don't have a
GPU.

We may also then have additional constraints when sharing buffers
between the display HW and video decode or even camera ISP HW.
Programmatically describing buffer allocation constraints is very
difficult and I'm not sure you can actually do it - there's some
pretty complex constraints out there! E.g. I believe there's a
platform where Y and UV planes of the reference frame need to be in
separate DRAM banks for real-time 1080p decode, or something like
that?

Anyway, I guess my point is that even if we solve how to allocate
buffers which will be shared between the GPU and display HW such that
both sets of constraints are satisfied, that may not be the end of
the story.

Cheers,

Tom