From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755708Ab2DWXAV (ORCPT ); Mon, 23 Apr 2012 19:00:21 -0400 Received: from mail-qa0-f42.google.com ([209.85.216.42]:57159 "EHLO mail-qa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751819Ab2DWXAS convert rfc822-to-8bit (ORCPT ); Mon, 23 Apr 2012 19:00:18 -0400 MIME-Version: 1.0 In-Reply-To: <1334757146-28335-1-git-send-email-daniel.vetter@ffwll.ch> References: <1334757146-28335-1-git-send-email-daniel.vetter@ffwll.ch> Date: Mon, 23 Apr 2012 16:00:17 -0700 X-Google-Sender-Auth: U4ctZ9XqYnjNKHXza9J1b73xJEk Message-ID: Subject: Re: [PATCH] dma-buf: mmap support From: Rebecca Schultz Zavin To: Daniel Vetter Cc: linaro-mm-sig@lists.linaro.org, LKML , DRI Development , linux-media@vger.kernel.org, Rob Clark Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I'd still rather see some form of explicit cache flusing api, but I can do that through my exporter (and probably will to get started). Otherwise this looks good to me except for comment inline. On Wed, Apr 18, 2012 at 6:52 AM, Daniel Vetter 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. > > Cc: Rob Clark > Cc: Rebecca Schultz Zavin > Signed-Off-by: Daniel Vetter > --- >  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..8a573b9 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; > + > +       /* 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; > + > +       dmabuf = file->private_data; Better move this above the if statement where you deref the pointer... > + > +       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 >