Re: [PATCH 0/3] dma-buf: Flag vmap'ed memory as system or I/O memory

[Sumit Semwal <sumit.semwal@linaro.org>]

Hi Thomas,

On Fri, 18 Sep 2020 at 11:36, Sumit Semwal <sumit.semwal@linaro.org> wrote:
>
> Hello Thomas,
>
> On Mon, 14 Sep 2020 at 16:55, Thomas Zimmermann <tzimmermann@suse.de> wrote:
> >
> > Dma-buf provides vmap() and vunmap() for retrieving 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.
>
> Thank you for the patchset - it is a really nice, clean bit to add!
> >
> > 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.
> >
> > [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 (3):
> >   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
>
> FWIW, for the series, please feel free to add my
> Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Of course, once the errors found by kernel test robot are fixed :).
>
> >
> >  Documentation/driver-api/dma-buf.rst          |   3 +
> >  drivers/dma-buf/dma-buf.c                     |  40 +++---
> >  drivers/gpu/drm/drm_gem_cma_helper.c          |  16 ++-
> >  drivers/gpu/drm/drm_gem_shmem_helper.c        |  17 ++-
> >  drivers/gpu/drm/drm_prime.c                   |  14 +-
> >  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 ++-
> >  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 ++-
> >  include/drm/drm_prime.h                       |   5 +-
> >  include/linux/dma-buf-map.h                   | 126 ++++++++++++++++++
> >  include/linux/dma-buf.h                       |  11 +-
> >  15 files changed, 274 insertions(+), 82 deletions(-)
> >  create mode 100644 include/linux/dma-buf-map.h
> >
> > --
> > 2.28.0
> >
>
> Best,
> Sumit.

Best,
Sumit.