Re: [RFC PATCH] drm/virtio: Export resource handles via DMA-buf API

[Daniel Vetter]

On Sat, Oct 05, 2019 at 02:41:54PM +0900, Tomasz Figa wrote:
> Hi Daniel, Gerd,
> 
> On Tue, Sep 17, 2019 at 10:23 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
> >
> > On Thu, Sep 12, 2019 at 06:41:21PM +0900, Tomasz Figa wrote:
> > > This patch is an early RFC to judge the direction we are following in
> > > our virtualization efforts in Chrome OS. The purpose is to start a
> > > discussion on how to handle buffer sharing between multiple virtio
> > > devices.
> > >
> > > On a side note, we are also working on a virtio video decoder interface
> > > and implementation, with a V4L2 driver for Linux. Those will be posted
> > > for review in the near future as well.
> > >
> > > Any feedback will be appreciated! Thanks in advance.
> > >
> > > ===
> > >
> > > With the range of use cases for virtualization expanding, there is going
> > > to be more virtio devices added to the ecosystem. Devices such as video
> > > decoders, encoders, cameras, etc. typically work together with the
> > > display and GPU in a pipeline manner, which can only be implemented
> > > efficiently by sharing the buffers between producers and consumers.
> > >
> > > Existing buffer management framework in Linux, such as the videobuf2
> > > framework in V4L2, implements all the DMA-buf handling inside generic
> > > code and do not expose any low level information about the buffers to
> > > the drivers.
> > >
> > > To seamlessly enable buffer sharing with drivers using such frameworks,
> > > make the virtio-gpu driver expose the resource handle as the DMA address
> > > of the buffer returned from the DMA-buf mapping operation. Arguably, the
> > > resource handle is a kind of DMA address already, as it is the buffer
> > > identifier that the device needs to access the backing memory, which is
> > > exactly the same role a DMA address provides for native devices.
> > >
> > > A virtio driver that does memory management fully on its own would have
> > > code similar to following. The code is identical to what a regular
> > > driver for real hardware would do to import a DMA-buf.
> > >
> > > static int virtio_foo_get_resource_handle(struct virtio_foo *foo,
> > >                                         struct dma_buf *dma_buf, u32 *id)
> > > {
> > >       struct dma_buf_attachment *attach;
> > >       struct sg_table *sgt;
> > >       int ret = 0;
> > >
> > >       attach = dma_buf_attach(dma_buf, foo->dev);
> > >       if (IS_ERR(attach))
> > >               return PTR_ERR(attach);
> > >
> > >       sgt = dma_buf_map_attachment(attach, DMA_BIDIRECTIONAL);
> > >       if (IS_ERR(sgt)) {
> > >               ret = PTR_ERR(sgt);
> > >               goto err_detach;
> > >       }
> > >
> > >       if (sgt->nents != 1) {
> > >               ret = -EINVAL;
> > >               goto err_unmap;
> > >       }
> > >
> > >       *id = sg_dma_address(sgt->sgl);
> >
> > I agree with Gerd, this looks pretty horrible to me.
> >
> > The usual way we've done these kind of special dma-bufs is:
> >
> > - They all get allocated at the same place, through some library or
> >   whatever.
> >
> > - You add a dma_buf_is_virtio(dma_buf) function, or maybe something that
> >   also upcasts or returns NULL, which checks for dma_buf->ops.
> >
> 
> Thanks for a lot of valuable feedback and sorry for the late reply.
> 
> While I agree that stuffing the resource ID in sg_dma_address() is
> quite ugly (for example, the regular address arithmetic doesn't work),
> I still believe we need to convey information about these buffers
> using regular kernel interfaces.
> 
> Drivers in some subsystems like DRM tend to open code any buffer
> management and then it wouldn't be any problem to do what you
> suggested. However, other subsystems have generic frameworks for
> buffer management, like videobuf2 for V4L2. Those assume regular
> DMA-bufs that can be handled with regular dma_buf_() API and described
> using sgtables and/or pfn vectors and/or DMA addresses.

"other subsystem sucks" doesn't sound like a good design paradigm to me.
Forced midlayers are a bad design decision isn't really new at all ...

> > - Once you've upcasted at runtime by checking for ->ops, you can add
> >   whatever fancy interfaces you want. Including a real&proper interface to
> >   get at whatever underlying id you need to for real buffer sharing
> >   between virtio devices.
> >
> > In a way virtio buffer/memory ids are a kind of private bus, entirely
> > distinct from the dma_addr_t bus. So can't really stuff them under this
> > same thing like we e.g. do with pci peer2peer.
> 
> As I mentioned earlier, there is no single "dma_addr_t bus". Each
> device (as in struct device) can be on its own different DMA bus, with
> a different DMA address space. There is not even a guarantee that a
> DMA address obtained for one PCI device will be valid for another if
> they are on different buses, which could have different address
> mappings.
> 
> Putting that aside, we're thinking about a different approach, as Gerd
> suggested in another thread, the one about the Virtio Video Decoder
> protocol. I'm going to reply there, making sure to CC everyone
> involved here.

ok.
-Daniel

> 
> Best regards,
> Tomasz
> 
> > -Daniel
> >
> > >
> > > err_unmap:
> > >       dma_buf_unmap_attachment(attach, sgt, DMA_BIDIRECTIONAL);
> > > err_detach:
> > >       dma_buf_detach(dma_buf, attach);
> > >
> > >       return ret;
> > > }
> > >
> > > On the other hand, a virtio driver that uses an existing kernel
> > > framework to manage buffers would not need to explicitly handle anything
> > > at all, as the framework part responsible for importing DMA-bufs would
> > > already do the work. For example, a V4L2 driver using the videobuf2
> > > framework would just call thee vb2_dma_contig_plane_dma_addr() function
> > > to get what the above open-coded function would return.
> > >
> > > Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > ---
> > >  drivers/gpu/drm/virtio/virtgpu_drv.c   |  2 +
> > >  drivers/gpu/drm/virtio/virtgpu_drv.h   |  4 ++
> > >  drivers/gpu/drm/virtio/virtgpu_prime.c | 81 ++++++++++++++++++++++++++
> > >  3 files changed, 87 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.c b/drivers/gpu/drm/virtio/virtgpu_drv.c
> > > index 0fc32fa0b3c0..ac095f813134 100644
> > > --- a/drivers/gpu/drm/virtio/virtgpu_drv.c
> > > +++ b/drivers/gpu/drm/virtio/virtgpu_drv.c
> > > @@ -210,6 +210,8 @@ static struct drm_driver driver = {
> > >  #endif
> > >       .prime_handle_to_fd = drm_gem_prime_handle_to_fd,
> > >       .prime_fd_to_handle = drm_gem_prime_fd_to_handle,
> > > +     .gem_prime_export = virtgpu_gem_prime_export,
> > > +     .gem_prime_import = virtgpu_gem_prime_import,
> > >       .gem_prime_get_sg_table = virtgpu_gem_prime_get_sg_table,
> > >       .gem_prime_import_sg_table = virtgpu_gem_prime_import_sg_table,
> > >       .gem_prime_vmap = virtgpu_gem_prime_vmap,
> > > diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
> > > index e28829661724..687cfce91885 100644
> > > --- a/drivers/gpu/drm/virtio/virtgpu_drv.h
> > > +++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
> > > @@ -367,6 +367,10 @@ void virtio_gpu_object_free_sg_table(struct virtio_gpu_object *bo);
> > >  int virtio_gpu_object_wait(struct virtio_gpu_object *bo, bool no_wait);
> > >
> > >  /* virtgpu_prime.c */
> > > +struct dma_buf *virtgpu_gem_prime_export(struct drm_gem_object *obj,
> > > +                                      int flags);
> > > +struct drm_gem_object *virtgpu_gem_prime_import(struct drm_device *dev,
> > > +                                             struct dma_buf *buf);
> > >  struct sg_table *virtgpu_gem_prime_get_sg_table(struct drm_gem_object *obj);
> > >  struct drm_gem_object *virtgpu_gem_prime_import_sg_table(
> > >       struct drm_device *dev, struct dma_buf_attachment *attach,
> > > diff --git a/drivers/gpu/drm/virtio/virtgpu_prime.c b/drivers/gpu/drm/virtio/virtgpu_prime.c
> > > index dc642a884b88..562eb1a2ed5b 100644
> > > --- a/drivers/gpu/drm/virtio/virtgpu_prime.c
> > > +++ b/drivers/gpu/drm/virtio/virtgpu_prime.c
> > > @@ -22,6 +22,9 @@
> > >   * Authors: Andreas Pokorny
> > >   */
> > >
> > > +#include <linux/dma-buf.h>
> > > +#include <linux/dma-direction.h>
> > > +
> > >  #include <drm/drm_prime.h>
> > >
> > >  #include "virtgpu_drv.h"
> > > @@ -30,6 +33,84 @@
> > >   * device that might share buffers with virtgpu
> > >   */
> > >
> > > +static struct sg_table *
> > > +virtgpu_gem_map_dma_buf(struct dma_buf_attachment *attach,
> > > +                     enum dma_data_direction dir)
> > > +{
> > > +     struct drm_gem_object *obj = attach->dmabuf->priv;
> > > +     struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
> > > +     struct sg_table *sgt;
> > > +     int ret;
> > > +
> > > +     sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
> > > +     if (!sgt)
> > > +             return ERR_PTR(-ENOMEM);
> > > +
> > > +     ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
> > > +     if (ret) {
> > > +             kfree(sgt);
> > > +             return ERR_PTR(-ENOMEM);
> > > +     }
> > > +
> > > +     sg_dma_address(sgt->sgl) = bo->hw_res_handle;
> > > +     sg_dma_len(sgt->sgl) = obj->size;
> > > +     sgt->nents = 1;
> > > +
> > > +     return sgt;
> > > +}
> > > +
> > > +static void virtgpu_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
> > > +                                   struct sg_table *sgt,
> > > +                                   enum dma_data_direction dir)
> > > +{
> > > +     sg_free_table(sgt);
> > > +     kfree(sgt);
> > > +}
> > > +
> > > +static const struct dma_buf_ops virtgpu_dmabuf_ops =  {
> > > +     .cache_sgt_mapping = true,
> > > +     .attach = drm_gem_map_attach,
> > > +     .detach = drm_gem_map_detach,
> > > +     .map_dma_buf = virtgpu_gem_map_dma_buf,
> > > +     .unmap_dma_buf = virtgpu_gem_unmap_dma_buf,
> > > +     .release = drm_gem_dmabuf_release,
> > > +     .mmap = drm_gem_dmabuf_mmap,
> > > +     .vmap = drm_gem_dmabuf_vmap,
> > > +     .vunmap = drm_gem_dmabuf_vunmap,
> > > +};
> > > +
> > > +struct dma_buf *virtgpu_gem_prime_export(struct drm_gem_object *obj,
> > > +                                      int flags)
> > > +{
> > > +     struct dma_buf *buf;
> > > +
> > > +     buf = drm_gem_prime_export(obj, flags);
> > > +     if (!IS_ERR(buf))
> > > +             buf->ops = &virtgpu_dmabuf_ops;
> > > +
> > > +     return buf;
> > > +}
> > > +
> > > +struct drm_gem_object *virtgpu_gem_prime_import(struct drm_device *dev,
> > > +                                             struct dma_buf *buf)
> > > +{
> > > +     struct drm_gem_object *obj;
> > > +
> > > +     if (buf->ops == &virtgpu_dmabuf_ops) {
> > > +             obj = buf->priv;
> > > +             if (obj->dev == dev) {
> > > +                     /*
> > > +                      * Importing dmabuf exported from our own gem increases
> > > +                      * refcount on gem itself instead of f_count of dmabuf.
> > > +                      */
> > > +                     drm_gem_object_get(obj);
> > > +                     return obj;
> > > +             }
> > > +     }
> > > +
> > > +     return drm_gem_prime_import(dev, buf);
> > > +}
> > > +
> > >  struct sg_table *virtgpu_gem_prime_get_sg_table(struct drm_gem_object *obj)
> > >  {
> > >       struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
> > > --
> > > 2.23.0.237.gc6a4ce50a0-goog
> > >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch