[PATCH v3 0/4] dma-buf: Flag vmap'ed memory as system or I/O memory

[Sam Ravnborg]

Hi Thomas.

Sorry for chiming in late here, have been offline for a while.

On Fri, Sep 25, 2020 at 01:55:57PM +0200, Thomas Zimmermann wrote:
> Dma-buf provides vmap() and vunmap() for retriving and releasing mappings
> of dma-buf memory in kernel address space. The functions operate with plain
> addresses and the assumption is that the memory can be accessed with load
> and store operations. This is not the case on some architectures (e.g.,
> sparc64) where I/O memory can only be accessed with dedicated instructions.
> 
> This patchset introduces struct dma_buf_map, which contains the address of
> a buffer and a flag that tells whether system- or I/O-memory instructions
> are required.

The whole idea with a struct that can represent both a pointer to system
memory and io memory is very nice.
dma-buf is one user of this but we may/will see other users. So the
naming seems of as this should be a concept independent of dma-buf.

And then the struct definition and all the helpers should be moved away
from dma-buf.

Maybe something like this:

struct simap {
       union {
               void __iomem *vaddr_iomem;
               void *vaddr;
       };
       bool is_iomem;
};

Where simap is a shorthand for system_iomem_map
And it could al be stuffed into a include/linux/simap.h file.

Not totally sold on the simap name - but wanted to come up with
something.

With this approach users do not have to pull in dma-buf to use it and
users will not confuse that this is only for dma-buf usage.

I am sorry for being late with the feedback.

	Sam


> Some background: updating the DRM framebuffer console on sparc64 makes the
> kernel panic. This is because the framebuffer memory cannot be accessed with
> system-memory instructions. We currently employ a workaround in DRM to
> address this specific problem. [1]
> 
> To resolve the problem, we'd like to address it at the most common point,
> which is the dma-buf framework. The dma-buf mapping ideally knows if I/O
> instructions are required and exports this information to it's users. The
> new structure struct dma_buf_map stores the buffer address and a flag that
> signals I/O memory. Affected users of the buffer (e.g., drivers, frameworks)
> can then access the memory accordingly.
> 
> This patchset only introduces struct dma_buf_map, and updates struct dma_buf
> and it's interfaces. Further patches can update dma-buf users. For example,
> there's a prototype patchset for DRM that fixes the framebuffer problem. [2]
> 
> Further work: TTM, one of DRM's memory managers, already exports an
> is_iomem flag of its own. It could later be switched over to exporting struct
> dma_buf_map, thus simplifying some code. Several DRM drivers expect their
> fbdev console to operate on I/O memory. These could possibly be switched over
> to the generic fbdev emulation, as soon as the generic code uses struct
> dma_buf_map.
> 
> v3:
> 	* update fastrpc driver (kernel test robot)
> 	* expand documentation (Daniel)
> 	* move documentation into separate patch
> v2:
> 	* always clear map parameter in dma_buf_vmap() (Daniel)
> 	* include dma-buf-heaps and i915 selftests (kernel test robot)
> 	* initialize cma_obj before using it in drm_gem_cma_free_object()
> 	  (kernel test robot)
> 
> [1] https://lore.kernel.org/dri-devel/20200725191012.GA434957@ravnborg.org/
> [2] https://lore.kernel.org/dri-devel/20200806085239.4606-1-tzimmermann@suse.de/
> 
> Thomas Zimmermann (4):
>   dma-buf: Add struct dma-buf-map for storing struct dma_buf.vaddr_ptr
>   dma-buf: Use struct dma_buf_map in dma_buf_vmap() interfaces
>   dma-buf: Use struct dma_buf_map in dma_buf_vunmap() interfaces
>   dma-buf: Document struct dma_buf_map
> 
>  Documentation/driver-api/dma-buf.rst          |   9 +
>  drivers/dma-buf/dma-buf.c                     |  42 ++--
>  drivers/dma-buf/heaps/heap-helpers.c          |  10 +-
>  drivers/gpu/drm/drm_gem_cma_helper.c          |  20 +-
>  drivers/gpu/drm/drm_gem_shmem_helper.c        |  17 +-
>  drivers/gpu/drm/drm_prime.c                   |  15 +-
>  drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c   |  13 +-
>  drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c    |  13 +-
>  .../drm/i915/gem/selftests/i915_gem_dmabuf.c  |  18 +-
>  .../gpu/drm/i915/gem/selftests/mock_dmabuf.c  |  14 +-
>  drivers/gpu/drm/tegra/gem.c                   |  23 ++-
>  .../common/videobuf2/videobuf2-dma-contig.c   |  17 +-
>  .../media/common/videobuf2/videobuf2-dma-sg.c |  19 +-
>  .../common/videobuf2/videobuf2-vmalloc.c      |  21 +-
>  drivers/misc/fastrpc.c                        |   6 +-
>  include/drm/drm_prime.h                       |   5 +-
>  include/linux/dma-buf-map.h                   | 193 ++++++++++++++++++
>  include/linux/dma-buf.h                       |  11 +-
>  18 files changed, 372 insertions(+), 94 deletions(-)
>  create mode 100644 include/linux/dma-buf-map.h
> 
> --
> 2.28.0