(Disclaimer: I come from a graphics background, so sorry if I use graphicsy
terminology; please let me know if any of this isn't clear. I tried.)
There is an wide range of hardware capabilities that require different
programming approaches in order to perform optimally. We need to define an
interface that is flexible enough to handle each of them, or else it won't be
used and we'll be right back where we are today: with vendors rolling their own
support for the things they need.
I'm going to try to enumerate some of the more unique usage patterns as
I see them here.
- Many or all engines may sit behind asynchronous command stream interfaces.
Programming is done through "batch buffers"; a set of commands operating on a
set of in-memory buffers is prepared and then submitted to the kernel to be
queued. The kernel will first make sure all of the buffers are resident
(which may require paging or mapping into an IOMMU/GART, a.k.a. "pinning"),
then queue the batch of commands. The hardware will process the commands at
its earliest convenience, and then interrupt the CPU to notify it that it's
done with the buffers (i.e. it can now be "unpinned").
Those familiar with graphics may recognize this programming model as a
classic GPU command stream. But it doesn't need to be used exclusively with
GPUs; any number of devices may have such an on-demand paging mechanism.
- In contrast, some engines may also stream to or from memory continuously
(e.g., video capture or scanout); such buffers need to be pinned for an
extended period of time, not tied to the command streams described above.
- There can be multiple different command streams working at the same time on
the same buffers. (There may be hardware synchronization primitives between
the multiple command streams so the CPU doesn't have to babysit too much, for
both performance and power reasons.)
- In some systems, IOMMU/GART may be much smaller than physical memory; older
GPUs and SoCs have this. To support these, we need to be able to map and
unmap pages into the IOMMU on demand in our host command stream flow. This
model also requires patching up pending batch buffers before queueing them to
the hardware, to update them to point to the newly-mapped location in the
IOMMU.
- In other systems, IOMMU/GART may be much larger than physical memory; more
modern GPUs and SoCs have this. With these, we can reserve virtual (IOMMU)
address space for each buffer up front. To userspace, the buffers always
appear "mapped". This is similar in concept to how the CPU virtual space in
userspace sticks around even when the underlying memory is paged out to disk.
In this case, pinning is performed at the same time as the small-IOMMU case
above, but in the normal/fast case, the pages are never paged out of the
IOMMU, and the pin step just increments a refcount to prevent the pages from
being evicted.
It is desirable to keep the same IOMMU address for:
a) implementing features such as
http://www.opengl.org/registry/specs/NV/shader_buffer_load.txt
(OpenGL client applications and shaders manipulate GPU vaddr pointers
directly; a GPU virtual address is assumed to be valid forever).
b) performance: scanning through the command buffers to patch up pointers can
be very expensive.
One other important note: buffer format properties may be necessary to set up
mappings (both CPU and iommu mappings). For example, both types of mappings
may need to know tiling properties of the buffer. This may be a property of
the mapping itself (consider it baked into the page table entries), not
necessarily something a different driver or userspace can program later
independently.
Some of the discussion I heard this morning tended towards being overly
simplistic and didn't seem to cover each of these cases well. Hopefully this
will help get everyone on the same page.
Thanks,
Robert
Hi all,
A bit later than what I've hoped for, but here we go [Jesse and Dave,
please correct/clarify/extend where you see fit]:
The core idea of GEM is to identify graphic buffer objects with 32bit ids.
The reason being "X runs out of open fds" (KDE easily reaches a few
thousand).
The core design principle behind GEM is that the kernel is in full control
of the allocation of these buffer objects and is free to move the around
in any way it sees fit. This is to make concurrent rendering by multiple
processes possible while userspace can still assume that it is in sole
possession of the gpu - GEM means "graphics execution manager".
Below some more details on what GEM is and does, what it does
_not_ do and how it relates to other graphic subsystems.
GEM does ...
------------
- lifecycle management. Userspace references are associated with the drm
fd and get reaped on close (in case userspace forgets about them).
- per-device global names to exchange buffers between processes (eg dri2).
These names are again 32bit ids. These global ids do not count as
userspace references and don't prevent a buffer from being reaped.
- it implements very few generic ioctls:
* flink for creating a global name for a buffer object
* open for getting a per-fd handle to a buffer object with a global name
* close for dropping a per-fd handle.
- a little bit of kernel-internal helpers to facilitate mmap (by blending
multiple buffer objects into the single drm device address space) and a
few other things.
That's it, i.e. GEM is very much meant to be as simple as possible.
Driver-specific GEM ioctls
--------------------------
The generic GEM stuff is obviously not very useful. So drivers implement
quite a bit driver-specific ioctls, like:
- buffer creation. In recent kernels there is some support to create dumb
scanout objects for KMS. But they're only really useful for boot-splashs
and unaccelerated dumb KMS drivers. Creating buffers usable for
rendering is only possible with driver specific ioctls.
- command submission. An important part is mapping abstract buffer ids to
actual gpu address (and rewriting batchbuffers with these). In the
future, with support for virtual gpu address spaces this might change.
- tiling management. The kernel needs to know this to correctly
tile/detile buffers when moving them around (e.g. evicting from vram).
- command completion signalling and gpu/cpu synchronization.
There are currently two approaches for implementing a GEM driver:
- roll-your-own, used by drm/i915 (and sometimes getting flaked for NIH).
- ttm-base: radeon & nouveau.
GEM does not ...
----------------
This still leaves out a few things that I've seen mentioned as
ideas/requirements here and elsewhere:
- cross-device buffer sharing and namespaces (see below) and
- buffer format handling and mediation between different users (except
tiling as mentioned above). The reason here is that gpus are a mess
and one of the worst parts is format handling. Better keep that out
of the kernel ...
KMS (kernel mode setting)
-------------------------
KMS is essentially just a port of the xrandr api to the kernel as an ioctl
interface:
- crtcs feed (possible multiple) outputs and get their data from a
framebuffer object. A major part of KMS is also the support for
vsynced-pageflipping of framebuffers.
- Internally there's some support infrastructure to simplify drivers (all
the drm_*_helper.c code).
- framebuffers are created from a opaque driver-specific 32bit id and a
format description. For GEM drivers these ids name GEM objects, but that
need not be: The recently merged qemu kms driver does not implement gem
and has one unique buffer object with id 0.
- as mentioned above there newly is a generic ioctl to create an object
suitable as a dumb scanout (plus some support to mmap it).
- currently KMS has no generic support for overlays (there are
driver-specific ioctls in i915 and vmgfx, though). Jesse Barnes has
posted an RFC to remedy this:
http://www.mail-archive.com/dri-devel@lists.freedesktop.org/msg10415.html
GEM and PRIME
-------------
PRIME is a proof-of-concept implementation from Dave Airlie for sharing
GEM objects between drivers/devices: Buffer sharing is done with a list of
struct page pointers. While being shared, buffers can't be moved anymore.
No further buffer description is passed along in the kernel, format/layout
mediation is to be handled in userspace.
Blog-post describing the initial design for sharing buffers between an
integrated Intel igd and a discrete ATI gpu:
http://airlied.livejournal.com/71734.html
Other code using the same framework to render on an Intel igd and display
the framebuffer on an usb-connected displayport:
http://git.kernel.org/?p=linux/kernel/git/airlied/drm-testing.git;a=shortlo…
GEM/KMS and fbdev
-----------------
There's some minimal support to emulate an fbdev with a gem/kms driver.
Resolution can't be changed and it's unaccelerated. There's been some
muttering once in a while to better integrate this with either a kms
kernel console driver or by routing fbdev resolution changes to kms.
But the main use case is to display a kernel oops, which works. For
everything else there's X (or an EGL client that understands kms).
-Daniel
--
Daniel Vetter
Mail: daniel(a)ffwll.ch
Mobile: +41 (0)79 365 57 48
Hi!
I just want to clarify some buffer object operations and terminology that seems confusing to people and that are used by most modern GPU drivers.
I think it's useful to be aware of this, going forward in the memory manager discussions.
Terminology:
Scanout buffer: Buffer that is used for continous access by a device. Needs to be permanently pinned.
Pinned buffer: A pinned buffer may not move and may not change backing pages. Allows it to be mapped to a device.
Synchronization object: An object that is either in a signaled or non-signaled state. Signaled means that the device is done with the buffer, and has flushed its caches. A synchronization object has a device-specific part that may, for example, contain flushing state.
Basic device use of a buffer:
Scanout buffers (and perhaps also capture buffers?) are typically pinned.
Other buffers that are temporarily used by a GPU and, for example, a video decoding engine or image processor are typically *not* pinned. The usage pattern for submitting any commands that affect the buffer is as follows:
1) Take a mutex that stops the buffer from being moved. This mutex could be global (stops all buffers from being moved) or per-buffer.
2) Wait on any previous synchronization objects attached to the buffer, if those sync objects would not be implicitly signaled when the device executes its work. This is where it becomes bad to have a global mutex under 1).
3) Validate the buffer. This means setting up any missing (contigous) device mappings or move to VRAM, flush cpu caches if necessary.
4) Patch up the device commands to reflect any movement of the buffer in 3). New offsets, SG-lists etc.
5) Submit the device commands.
6) Create a new synchronization object and attach it to the buffer.
7) Release the mutex taken i 1).
The buffer will not be moved until the synchronization object has signaled, and mappings set up under 3) will not be torn down until the memory manager receives a request to free up mapping resources.
I'd call this "Generation 2" device buffer management. (Intel (uses busy lists, no sync objects), Radeon, Nouveau, vmwgfx, New VIA)
"Generation 1" was using a global memory manager for pinned buffers (SiS, old VIA DRM drivers)
Generation 3 would be page based device MMUs with programmable apertures to access VRAM.
What we were discussing today is basically creating a unified gen 1 manager, with a new user-space interface.
/Thomas
DRM support for platform devices dropped last year and was drastically
improved earlier this year. Qualcomm uses it for a really weak DRM driver
that handles memory for X but does GPU and display through a different
interface. Feel free to flame me for that.. :).
https://www.codeaurora.org/gitweb/quic/la/?p=kernel/msm.git;a=blob;f=driver…
And I believe OMAP also has a solution somewhere (sorry, I couldn't find a URL).
Jordan
Hi!
Just wanted to share some thoughts about CMA, TTM and the problems with
conflicting mappings.
1) CMA seems to be a nice way to handle the problem with contigous
pages, although it seems Arnd has some concerns. It would be nice to
here about those. Some thoughts:
a) It seems fairly straightforward to interface CMA with TTM. The
benefit would be that CMA could ask TTM at any time (using a shrinker
callback) to release its contigous pages, and TTM would do so once the
GPU is finished with them (unless of course they are used with a pinned
buffer object, like a scanout buffer). CMA would need to be extended
with a small API to create / free a contigous range and to actually
populate that range with pages.
b) DRM, TTM and it seems CMA all need a range allocator. There is a
reasonable implementation in drm_mm.c, which since the original
implementation has seen a fair bit of improvement. Should we try to move
that to linux/lib ?
c) Could the CMA technique be used also to keep a pool of pages that are
unmapped from the linear kernel map? Essentially a range of HIGHMEM
pages? The benefit compared to just having a pool of HIGHMEM pages by
itself would be that the graphics subsystem would have priority over
normal system use (moving out movable contents), and could use these
pages with nonstandard caching attribute maps if needed. If this is
considered a good idea, we could perhaps consider placing the default
CMA region in HIGHMEM.
/Thomas
Hi all,
I've updated the mini-summit wiki with a couple more details:
https://wiki.linaro.org/Events/2011-05-MM
one of which is a sample use case description from Samsung. I would
encourage everyone to look at that and see if there are other use
cases they think would make more sense, or if there is clarification
or amendment of the current proposal. The discussion around this is
slated for Tuesday, so we have some time before it comes up in the
summit. As we proceed, I'll be moving sections of the agenda over
onto the discussion blueprints on launchpad (as that's how Linaro
tracks stuff), but everything will also be available on the wiki as
well as this list for those that can't or don't want to use launchpad.
Also, there are still a few components without representatives in the
summit, and while it would be nice to be able to have those in the
early parts of the sessions, I would rather flex the agenda than omit
them. Even if there is no presentation on a component at the summit,
it would still be good to have written overviews of those, so I'll ask
again for volunteers.
cheers,
Jesse
Hey
This is a quick heads up that MM summit starts at *2pm* today (in 15mn)
and not at 3pm. The schedule is incorrect because we can't overlap the
plenaries, but discussion will start at 2pm.
See you there!
--
Loïc Minier
Hi all,
I just wanted to remind everyone about the room change and the fact
that the scheduler wouldn't let me show the session as starting at
1400, due to the plenaries. See you all soon.
cheers,
jesse
Hi,
>From the today's V4L presentation, there were two missing topics that may be
useful to include for our discussions:
a) V4L overlay mode;
b) dvb.
So, I'm bringing those two topics for discussions. If needed, I can do some
presentation about them, but it seemed better to start the discussion via
ML, in order to know more about the interests over those two subject.
a) V4L overlay mode
================
The V4L Overlay mode were used a lot during kernel 2.2 and 2.4 days, were
most hardware were not capable enough to do real-time processing of video
streams. It is supported by xawtv and a Xorg v4l driver, and uses XV overlay
extensions to display video. It is simple to setup and it requires no CPU
usage for it, as the video framebuffer is passed directly to the video hardware,
that programs DMA to write directly into the fb memory.
The main structures used on overlay mode (from kerrnel include/linux/videodev2.h)
are:
struct v4l2_pix_format {
__u32 width;
__u32 height;
__u32 pixelformat;
enum v4l2_field field;
__u32 bytesperline; /* for padding, zero if unused */
__u32 sizeimage;
enum v4l2_colorspace colorspace;
__u32 priv; /* private data, depends on pixelformat */
};
struct v4l2_framebuffer {
__u32 capability;
__u32 flags;
/* FIXME: in theory we should pass something like PCI device + memory
* region + offset instead of some physical address */
void *base;
struct v4l2_pix_format fmt;
};
/* Flags for the 'capability' field. Read only */
#define V4L2_FBUF_CAP_EXTERNOVERLAY 0x0001
#define V4L2_FBUF_CAP_CHROMAKEY 0x0002
#define V4L2_FBUF_CAP_LIST_CLIPPING 0x0004
#define V4L2_FBUF_CAP_BITMAP_CLIPPING 0x0008
#define V4L2_FBUF_CAP_LOCAL_ALPHA 0x0010
#define V4L2_FBUF_CAP_GLOBAL_ALPHA 0x0020
#define V4L2_FBUF_CAP_LOCAL_INV_ALPHA 0x0040
#define V4L2_FBUF_CAP_SRC_CHROMAKEY 0x0080
/* Flags for the 'flags' field. */
#define V4L2_FBUF_FLAG_PRIMARY 0x0001
#define V4L2_FBUF_FLAG_OVERLAY 0x0002
#define V4L2_FBUF_FLAG_CHROMAKEY 0x0004
#define V4L2_FBUF_FLAG_LOCAL_ALPHA 0x0008
#define V4L2_FBUF_FLAG_GLOBAL_ALPHA 0x0010
#define V4L2_FBUF_FLAG_LOCAL_INV_ALPHA 0x0020
#define V4L2_FBUF_FLAG_SRC_CHROMAKEY 0x0040
Using it is as simple as selecting a format that the video display framebuffer
supports, and send a couple of ioctls to the video adapter.
This is what the Xorg v4l driver (v4l.c) does (simplified, to ease
comprehension):
struct v4l2_framebuffer yuv_fbuf;
int on = 1;
if (-1 == ioctl(V4L_FD, VIDIOC_G_FBUF, &yuv_fbuf))
return;
/* Sets the Framebuf data: width, heigth, bpp, format, base and display position */
yuv_fbuf.fmt.width = surface->width;
yuv_fbuf.fmt.height = surface->height;
yuv_fbuf.fmt.bytesperline = surface->pitches[0];
yuv_fbuf.fmt.pixelformat = V4L2_PIX_FMT_YUYV;
yuv_fbuf.base = (char *)(memPhysBase + surface->offsets[0]);
memset(&yuv_win, 0, sizeof(yuv_win));
yuv_win.w.left = 0;
yuv_win.w.top = 0;
yuv_win.w.width = surface->width;
yuv_win.w.height = surface->height;
if (-1 == ioctl(V4L_FD, VIDIOC_S_FBUF, yuv_fbuf))
return;
/* Sets mem transfer type to overlay mode */
memset(&fmt, 0, sizeof(fmt));
fmt.type = V4L2_BUF_TYPE_VIDEO_OVERLAY;
if (-1 == ioctl(V4L_FD, VIDIOC_S_FMT, &fmt))
return;
/* Enables overlay mode. Data are transfered directly from video capture device into display framebuffer */
memcpy(&fmt.fmt.win, &pPPriv->yuv_win, sizeof(pPPriv->yuv_win));
if (-1 == ioctl(V4L_FD, VIDIOC_OVERLAY, &on))
return;
The main issue with the overlay mode, as discussed on the first day,
is that the framebuffer pointer is a physical address. The original
idea, on v4l2, were to use some framebuffer ID.
That's said, it wouldn't be hard to add a new flag at v4l2_framebuffer.flags,
saying meant to say that it should use a GEM ID. I had some discussions
with David Arlie about that when I've submitted the v4l driver fixes due to
the removal of the V4L1 old API. I'm planning to submit something like that in
the future, when I have some spare time for doing it. Eventually, if Linaro
is interested, it could be an interesting project, as it may solve some of
the current needs.
It is probably simpler to do that than to add another mode to the V4L MMAP stuff.
b) DVB
===
Several new ARM devices are now shipped with Digital TV integrated on that. On my
Country, we have several mobile phones, tablets and GPS devices with DTV receptors
inside. Modern TV sets and set-top-boxes already use Linux with DVB support inside.
GoogleTV will for sure need DTV support, as well as similar products.
Even being used everywhere, currently, no big vendor tried to send us patches to
improve their DVB support, but I suspect that this should happen soon. This is
just an educated guess. It would be nice to have some feedback about that from the
vendors.
The DVB API is completely different from the V4L one, and there are two different
types of DVB devices:
- Full-featured DVB devices, with MPEG-TS, audio and video codec inside it;
- "simple" devices that just provide a read() interface to get an MPEG-TS stream.
As modern ARM SoC devices can have a codec DSP processor, it makes sense for them
to use the full-featured API, providing just audio and video via the DVB API
(yes, DVB has a different way to control and export audio/video than V4L/alsa).
The question here is: is there any demand for it right now? If so, what are the
requirements? Are the memory management requirements identical to the current
ones?
Thanks,
Mauro
