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

From: [email-masked] (Tom Cooksey)
Date: Tue, 6 Aug 2013 15:03:56 +0100

Hi Rob,

> >> > 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?
> >>
> >> yes, this was discussed.  This is different from pitch/format/size
> >> constraints.. it is really just a placement constraint (ie. where 
> >> do the physical pages go).  IIRC the conclusion was to use a dummy
> >> devices with it's own CMA pool for attaching the Y vs UV buffers.
> >>
> >> > 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.
> >> >
> >>
> >> that was part of the reason to punt this problem to userspace ;-)
> >>
> >> In practice, the kernel drivers doesn't usually know too much about
> >> the dimensions/format/etc.. that is really userspace level 
> >> knowledge. There are a few exceptions when the kernel needs to know
> >> how to setup GTT/etc for tiled buffers, but normally this sort of 
> >> information is up at the next level up (userspace, and 
> >> drm_framebuffer in case of scanout).  Userspace media frameworks 
> >> like GStreamer already have a concept of format/caps negotiation.  
> >> For non-display<->gpu sharing, I think this is probably where this 
> >> sort of constraint negotiation should be handled.
> >
> > I agree that user-space will know which devices will access the
> > buffer and thus can figure out at least a common pixel format. 
> > Though I'm not so sure userspace can figure out more low-level 
> > details like alignment and placement in physical memory, etc.
> > 
> 
> well, let's divide things up into two categories:
> 
> 1) the arrangement and format of pixels.. ie. what userspace would
> need to know if it mmap's a buffer.  This includes pixel format,
> stride, etc.  This should be negotiated in userspace, it would be
> crazy to try to do this in the kernel.

Absolutely. Pixel format has to be negotiated by user-space as in
most cases, user-space can map the buffer and thus will need to
know how to interpret the data.

> 2) the physical placement of the pages.  Ie. whether it is contiguous
> or not.  Which bank the pages in the buffer are placed in, etc.  This
> is not visible to userspace.

Seems sensible to me.

> ... This is the purpose of the attach step,
> so you know all the devices involved in sharing up front before
> allocating the backing pages. (Or in the worst case, if you have a
> "late attacher" you at least know when no device is doing dma access
> to a buffer and can reallocate and move the buffer.)  A long time
> back, I had a patch that added a field or two to 'struct
> device_dma_parameters' so that it could be known if a device required
> contiguous buffers.. looks like that never got merged, so I'd need to
> dig that back up and resend it.  But the idea was to have the 'struct
> device' encapsulate all the information that would be needed to
> do-the-right-thing when it comes to placement.

As I understand it, it's up to the exporting device to allocate the
memory backing the dma_buf buffer. I guess the latest possible point
you can allocate the backing pages is when map_dma_buf is first 
called? At that point the exporter can iterate over the current set
of attachments, programmatically determine the all the constraints of
all the attached drivers and attempt to allocate the backing pages
in such a way as to satisfy all those constraints?

Didn't you say that programmatically describing device placement
constraints was an unbounded problem? I guess we would have to
accept that it's not possible to describe all possible constraints
and instead find a way to describe the common ones?

One problem with this is it duplicates a lot of logic in each
driver which can export a dma_buf buffer. Each exporter will need to
do pretty much the same thing: iterate over all the attachments,
determine of all the constraints (assuming that can be done) and
allocate pages such that the lowest-common-denominator is satisfied.

Perhaps rather than duplicating that logic in every driver, we could
Instead move allocation of the backing pages into dma_buf itself?

> > Anyway, assuming user-space can figure out how a buffer should be
> > stored in memory, how does it indicate this to a kernel driver and
> > actually allocate it? Which ioctl on which device does user-space
> > call, with what parameters? Are you suggesting using something like
> > ION which exposes the low-level details of how buffers are laid out
> in
> > physical memory to userspace? If not, what?
> 
> no, userspace should not need to know this.  And having a central
> driver that knows this for all the other drivers in the system doesn't
> really solve anything and isn't really scalable.  At best you might
> want, in some cases, a flag you can pass when allocating.  For
> example, some of the drivers have a 'SCANOUT' flag that can be passed
> when allocating a GEM buffer, as a hint to the kernel that 'if this hw
> requires contig memory for scanout, allocate this buffer contig'.  But
> really, when it comes to sharing buffers between devices, we want this
> sort of information in dev->dma_params of the importing device(s).
   
If you had a single driver which knew the constraints of all devices
on that particular SoC and the interface allowed user-space to specify
which devices a buffer is intended to be used with, I guess it could
pretty trivially allocate pages which satisfy those constraints? It
wouldn't need a way to programmatically describe the constraints
either: As you say, if userspace sets the "SCANOUT" flag, it would
just "know" that on this SoC, that buffer needs to be physically 
contiguous for example.

Though It would effectively mean you'd need an "allocation" driver per
SoC, which as you say may not be scalable?

Cheers,

Tom