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

[Thomas Zimmermann]

Hi

Am 16.09.20 um 11:37 schrieb Daniel Vetter:
> On Mon, Sep 14, 2020 at 01:25:18PM +0200, Thomas Zimmermann 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.
>>
>> 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/
> 
> lgtm, imo ready to convert the follow-up patches over to this. But I think
> would be good to get at least some ack from the ttm side for the overall
> plan.

Yup, it would be nice if TTM could had out these types automatically.
Then all TTM-based drivers would automatically support it.

> 
> Also, I think we should put all the various helpers (writel/readl, memset,
> memcpy, whatever else) into the dma-buf-map.h helper, so that most code
> using this can just treat it as an abstract pointer type and never look
> underneath it.

We have some framebuffer helpers that rely on pointer arithmetic, so
we'd need that too. No big deal wrt code, but I was worried about the
overhead. If a loop goes over framebuffer memory, there's an if/else
branch for each access to the memory buffer.

Best regards
Thomas

> -Daniel
> 
>>
>> 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
>>
>>  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
>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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