Re: [PATCH] dma-buf: mmap support

Rob Clark
2012-05-11T15:30:51Z

On Tue, Apr 24, 2012 at 4:08 AM, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> Compared to Rob Clark's RFC I've ditched the prepare/finish hooks
> and corresponding ioctls on the dma_buf file. The major reason for
> that is that many people seem to be under the impression that this is
> also for synchronization with outstanding asynchronous processsing.
> I'm pretty massively opposed to this because:
>
> - It boils down reinventing a new rather general-purpose userspace
>  synchronization interface. If we look at things like futexes, this
>  is hard to get right.
> - Furthermore a lot of kernel code has to interact with this
>  synchronization primitive. This smells a look like the dri1 hw_lock,
>  a horror show I prefer not to reinvent.
> - Even more fun is that multiple different subsystems would interact
>  here, so we have plenty of opportunities to create funny deadlock
>  scenarios.
>
> I think synchronization is a wholesale different problem from data
> sharing and should be tackled as an orthogonal problem.
>
> Now we could demand that prepare/finish may only ensure cache
> coherency (as Rob intended), but that runs up into the next problem:
> We not only need mmap support to facilitate sw-only processing nodes
> in a pipeline (without jumping through hoops by importing the dma_buf
> into some sw-access only importer), which allows for a nicer
> ION->dma-buf upgrade path for existing Android userspace. We also need
> mmap support for existing importing subsystems to support existing
> userspace libraries. And a loot of these subsystems are expected to
> export coherent userspace mappings.
>
> So prepare/finish can only ever be optional and the exporter /needs/
> to support coherent mappings. Given that mmap access is always
> somewhat fallback-y in nature I've decided to drop this optimization,
> instead of just making it optional. If we demonstrate a clear need for
> this, supported by benchmark results, we can always add it in again
> later as an optional extension.
>
> Other differences compared to Rob's RFC is the above mentioned support
> for mapping a dma-buf through facilities provided by the importer.
> Which results in mmap support no longer being optional.
>
> Note that this dma-buf mmap patch does _not_ support every possible
> insanity an existing subsystem could pull of with mmap: Because it
> does not allow to intercept pagefaults and shoot down ptes importing
> subsystems can't add some magic of their own at these points (e.g. to
> automatically synchronize with outstanding rendering or set up some
> special resources). I've done a cursory read through a few mmap
> implementions of various subsytems and I'm hopeful that we can avoid
> this (and the complexity it'd bring with it).
>
> Additonally I've extended the documentation a bit to explain the hows
> and whys of this mmap extension.
>
> In case we ever want to add support for explicitly cache maneged
> userspace mmap with a prepare/finish ioctl pair, we could specify that
> userspace needs to mmap a different part of the dma_buf, e.g. the
> range starting at dma_buf->size up to dma_buf->size*2. This works
> because the size of a dma_buf is invariant over it's lifetime. The
> exporter would obviously need to fall back to coherent mappings for
> both ranges if a legacy clients maps the coherent range and the
> architecture cannot suppor conflicting caching policies. Also, this
> would obviously be optional and userspace needs to be able to fall
> back to coherent mappings.
>
> v2:
> - Spelling fixes from Rob Clark.
> - Compile fix for !DMA_BUF from Rob Clark.
> - Extend commit message to explain how explicitly cache managed mmap
>  support could be added later.
> - Extend the documentation with implementations notes for exporters
>  that need to manually fake coherency.
>
> v3:
> - dma_buf pointer initialization goof-up noticed by Rebecca Schultz
>  Zavin.
>
> Cc: Rob Clark <rob.clark@linaro.org>
> Cc: Rebecca Schultz Zavin <rebecca@android.com>
> Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Acked-by: Rob Clark <rob.clark@linaro.org>

> ---
>  Documentation/dma-buf-sharing.txt |   98 ++++++++++++++++++++++++++++++++++---
>  drivers/base/dma-buf.c            |   64 +++++++++++++++++++++++-
>  include/linux/dma-buf.h           |   16 ++++++
>  3 files changed, 170 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> index 3bbd5c5..5ff4d2b 100644
> --- a/Documentation/dma-buf-sharing.txt
> +++ b/Documentation/dma-buf-sharing.txt
> @@ -29,13 +29,6 @@ The buffer-user
>    in memory, mapped into its own address space, so it can access the same area
>    of memory.
>
> -*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
> -For this first version, A buffer shared using the dma_buf sharing API:
> -- *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
> -  this framework.
> -- with this new iteration of the dma-buf api cpu access from the kernel has been
> -  enable, see below for the details.
> -
>  dma-buf operations for device dma only
>  --------------------------------------
>
> @@ -313,6 +306,83 @@ Access to a dma_buf from the kernel context involves three steps:
>                                  enum dma_data_direction dir);
>
>
> +Direct Userspace Access/mmap Support
> +------------------------------------
> +
> +Being able to mmap an export dma-buf buffer object has 2 main use-cases:
> +- CPU fallback processing in a pipeline and
> +- supporting existing mmap interfaces in importers.
> +
> +1. CPU fallback processing in a pipeline
> +
> +   In many processing pipelines it is sometimes required that the cpu can access
> +   the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
> +   the need to handle this specially in userspace frameworks for buffer sharing
> +   it's ideal if the dma_buf fd itself can be used to access the backing storage
> +   from userspace using mmap.
> +
> +   Furthermore Android's ION framework already supports this (and is otherwise
> +   rather similar to dma-buf from a userspace consumer side with using fds as
> +   handles, too). So it's beneficial to support this in a similar fashion on
> +   dma-buf to have a good transition path for existing Android userspace.
> +
> +   No special interfaces, userspace simply calls mmap on the dma-buf fd.
> +
> +2. Supporting existing mmap interfaces in exporters
> +
> +   Similar to the motivation for kernel cpu access it is again important that
> +   the userspace code of a given importing subsystem can use the same interfaces
> +   with a imported dma-buf buffer object as with a native buffer object. This is
> +   especially important for drm where the userspace part of contemporary OpenGL,
> +   X, and other drivers is huge, and reworking them to use a different way to
> +   mmap a buffer rather invasive.
> +
> +   The assumption in the current dma-buf interfaces is that redirecting the
> +   initial mmap is all that's needed. A survey of some of the existing
> +   subsystems shows that no driver seems to do any nefarious thing like syncing
> +   up with outstanding asynchronous processing on the device or allocating
> +   special resources at fault time. So hopefully this is good enough, since
> +   adding interfaces to intercept pagefaults and allow pte shootdowns would
> +   increase the complexity quite a bit.
> +
> +   Interface:
> +      int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
> +                      unsigned long);
> +
> +   If the importing subsystem simply provides a special-purpose mmap call to set
> +   up a mapping in userspace, calling do_mmap with dma_buf->file will equally
> +   achieve that for a dma-buf object.
> +
> +3. Implementation notes for exporters
> +
> +   Because dma-buf buffers have invariant size over their lifetime, the dma-buf
> +   core checks whether a vma is too large and rejects such mappings. The
> +   exporter hence does not need to duplicate this check.
> +
> +   Because existing importing subsystems might presume coherent mappings for
> +   userspace, the exporter needs to set up a coherent mapping. If that's not
> +   possible, it needs to fake coherency by manually shooting down ptes when
> +   leaving the cpu domain and flushing caches at fault time. Note that all the
> +   dma_buf files share the same anon inode, hence the exporter needs to replace
> +   the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
> +   requred. This is because the kernel uses the underlying inode's address_space
> +   for vma tracking (and hence pte tracking at shootdown time with
> +   unmap_mapping_range).
> +
> +   If the above shootdown dance turns out to be too expensive in certain
> +   scenarios, we can extend dma-buf with a more explicit cache tracking scheme
> +   for userspace mappings. But the current assumption is that using mmap is
> +   always a slower path, so some inefficiencies should be acceptable.
> +
> +   Exporters that shoot down mappings (for any reasons) shall not do any
> +   synchronization at fault time with outstanding device operations.
> +   Synchronization is an orthogonal issue to sharing the backing storage of a
> +   buffer and hence should not be handled by dma-buf itself. This is explictly
> +   mentioned here because many people seem to want something like this, but if
> +   different exporters handle this differently, buffer sharing can fail in
> +   interesting ways depending upong the exporter (if userspace starts depending
> +   upon this implicit synchronization).
> +
>  Miscellaneous notes
>  -------------------
>
> @@ -336,6 +406,20 @@ Miscellaneous notes
>   the exporting driver to create a dmabuf fd must provide a way to let
>   userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
>
> +- If an exporter needs to manually flush caches and hence needs to fake
> +  coherency for mmap support, it needs to be able to zap all the ptes pointing
> +  at the backing storage. Now linux mm needs a struct address_space associated
> +  with the struct file stored in vma->vm_file to do that with the function
> +  unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
> +  with the anon_file struct file, i.e. all dma_bufs share the same file.
> +
> +  Hence exporters need to setup their own file (and address_space) association
> +  by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
> +  callback. In the specific case of a gem driver the exporter could use the
> +  shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
> +  zap ptes by unmapping the corresponding range of the struct address_space
> +  associated with their own file.
> +
>  References:
>  [1] struct dma_buf_ops in include/linux/dma-buf.h
>  [2] All interfaces mentioned above defined in include/linux/dma-buf.h
> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
> index 07cbbc6..7cfb405 100644
> --- a/drivers/base/dma-buf.c
> +++ b/drivers/base/dma-buf.c
> @@ -44,8 +44,26 @@ static int dma_buf_release(struct inode *inode, struct file *file)
>        return 0;
>  }
>
> +static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
> +{
> +       struct dma_buf *dmabuf;
> +
> +       if (!is_dma_buf_file(file))
> +               return -EINVAL;
> +
> +       dmabuf = file->private_data;
> +
> +       /* check for overflowing the buffer's size */
> +       if (vma->vm_pgoff + ((vma->vm_end - vma->vm_start) >> PAGE_SHIFT) >
> +           dmabuf->size >> PAGE_SHIFT)
> +               return -EINVAL;
> +
> +       return dmabuf->ops->mmap(dmabuf, vma);
> +}
> +
>  static const struct file_operations dma_buf_fops = {
>        .release        = dma_buf_release,
> +       .mmap           = dma_buf_mmap_internal,
>  };
>
>  /*
> @@ -82,7 +100,8 @@ struct dma_buf *dma_buf_export(void *priv, const struct dma_buf_ops *ops,
>                          || !ops->unmap_dma_buf
>                          || !ops->release
>                          || !ops->kmap_atomic
> -                         || !ops->kmap)) {
> +                         || !ops->kmap
> +                         || !ops->mmap)) {
>                return ERR_PTR(-EINVAL);
>        }
>
> @@ -406,3 +425,46 @@ void dma_buf_kunmap(struct dma_buf *dmabuf, unsigned long page_num,
>                dmabuf->ops->kunmap(dmabuf, page_num, vaddr);
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_kunmap);
> +
> +
> +/**
> + * dma_buf_mmap - Setup up a userspace mmap with the given vma
> + * @dma_buf:   [in]    buffer that should back the vma
> + * @vma:       [in]    vma for the mmap
> + * @pgoff:     [in]    offset in pages where this mmap should start within the
> + *                     dma-buf buffer.
> + *
> + * This function adjusts the passed in vma so that it points at the file of the
> + * dma_buf operation. It alsog adjusts the starting pgoff and does bounds
> + * checking on the size of the vma. Then it calls the exporters mmap function to
> + * set up the mapping.
> + *
> + * Can return negative error values, returns 0 on success.
> + */
> +int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
> +                unsigned long pgoff)
> +{
> +       if (WARN_ON(!dmabuf || !vma))
> +               return -EINVAL;
> +
> +       /* check for offset overflow */
> +       if (pgoff + ((vma->vm_end - vma->vm_start) >> PAGE_SHIFT) < pgoff)
> +               return -EOVERFLOW;
> +
> +       /* check for overflowing the buffer's size */
> +       if (pgoff + ((vma->vm_end - vma->vm_start) >> PAGE_SHIFT) >
> +           dmabuf->size >> PAGE_SHIFT)
> +               return -EINVAL;
> +
> +       /* readjust the vma */
> +       if (vma->vm_file)
> +               fput(vma->vm_file);
> +
> +       vma->vm_file = dmabuf->file;
> +       get_file(vma->vm_file);
> +
> +       vma->vm_pgoff = pgoff;
> +
> +       return dmabuf->ops->mmap(dmabuf, vma);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_mmap);
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 3efbfc2..1f78d15 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -61,6 +61,10 @@ struct dma_buf_attachment;
>  *                This Callback must not sleep.
>  * @kmap: maps a page from the buffer into kernel address space.
>  * @kunmap: [optional] unmaps a page from the buffer.
> + * @mmap: used to expose the backing storage to userspace. Note that the
> + *       mapping needs to be coherent - if the exporter doesn't directly
> + *       support this, it needs to fake coherency by shooting down any ptes
> + *       when transitioning away from the cpu domain.
>  */
>  struct dma_buf_ops {
>        int (*attach)(struct dma_buf *, struct device *,
> @@ -92,6 +96,8 @@ struct dma_buf_ops {
>        void (*kunmap_atomic)(struct dma_buf *, unsigned long, void *);
>        void *(*kmap)(struct dma_buf *, unsigned long);
>        void (*kunmap)(struct dma_buf *, unsigned long, void *);
> +
> +       int (*mmap)(struct dma_buf *, struct vm_area_struct *vma);
>  };
>
>  /**
> @@ -167,6 +173,9 @@ void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
>  void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
>  void *dma_buf_kmap(struct dma_buf *, unsigned long);
>  void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
> +
> +int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
> +                unsigned long);
>  #else
>
>  static inline struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> @@ -248,6 +257,13 @@ static inline void dma_buf_kunmap(struct dma_buf *dmabuf,
>                                  unsigned long pnum, void *vaddr)
>  {
>  }
> +
> +static inline int dma_buf_mmap(struct dma_buf *dmabuf,
> +                              struct vm_area_struct *vma,
> +                              unsigned long pgoff)
> +{
> +       return -ENODEV;
> +}
>  #endif /* CONFIG_DMA_SHARED_BUFFER */
>
>  #endif /* __DMA_BUF_H__ */
> --
> 1.7.10
>
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