On Tue, Apr 24, 2012 at 4:08 AM, Daniel Vetter <daniel.vetter@xxxxxxxx> 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@xxxxxxxxxx> > Cc: Rebecca Schultz Zavin <rebecca@xxxxxxxxxxx> > Signed-Off-by: Daniel Vetter <daniel.vetter@xxxxxxxx> Acked-by: Rob Clark <rob.clark@xxxxxxxxxx> > --- > 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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-media" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
