[Linaro-mm-sig] [PATCH] RFC: dma-buf: userspace mmap support

[Rob Clark]

On Fri, Mar 16, 2012 at 5:50 AM, Marcus Lorentzon
<marcus.xm.lorentzon at stericsson.com> wrote:
> On 03/15/2012 02:32 AM, Rob Clark wrote:
>>
>> From: Rob Clark<rob at ti.com>
>> [snip]
>>
>> In all cases, the mmap() call is allowed to fail, and the associated
>> dma_buf_ops are optional (mmap() will fail if at least the mmap()
>> op is not implemented by the exporter, but in either case the
>> {prepare,finish}_access() ops are optional).
>
> I sort of understand this approach. It allowes some implementations
> (ARM/Android) to move forward. But how would an application act if mmap
> fails? What is the option? How can the application detect if mmap is
> possible or not? Or is this mmap only supposed to be used from device
> specific libs like libdrm-xx/libv4l2-xx/libgralloc?

The most sane fail path probably depends on the use case.  For
example, if you have some code that is writing subtitle overlay into
frames of video, then the fallback that makes sense is probably just
to skip the buffer.  In other cases, showing error message and exiting
gracefully might be what makes sense.

I expect it is most likely that a particular exporting device will
either support mmap or not.  But I expect there would at least some
special cases, for example in product kernels with secure/protected
content.

> Can mmap fail for one buffer, but not another? Can it fail for a buffer that
> have successfully been mmapped once before (except for the usual
> ENOMEM/EAGAIN etc)?

well, this is always possible regardless, for example if client
process virtual address space is exhausted.

>> For now the prepare/finish access ioctls are kept simple with no
>> argument, although there is possibility to add additional ioctls
>> (or simply change the existing ioctls from _IO() to _IOW()) later
>> to provide optimization to allow userspace to specify a region of
>> interest.
>
> I like the idea of simple, assume the worst, no args, versions of begin/end
> access. But once we move forward, I don't just like the region, but also
> access type (R/W). R/W info allows the driver to make cache management
> optimizations otherwise impossible. Like if CPU with no alloc-on-write just
> write, a write buffer flush is enough to switch to a HW read. And (at least
> on ARM) cache clean can be done for all cache for large areas, but
> invalidate has to be done line by line. Eliminating the need to do
> invalidate, especially if region is small, compared to invalidate entire
> buffer line by line can make a huge difference.
> But I would like these in a separate ioctl to keep the normal case simple.
> Maybe as a separate patch even.

yeah, I expect that there will be some proposals and discussions about
that, so I wanted to get basic support in first :-)

>>
>> For a final patch, dma-buf.h would need to be split into what is
>> exported to userspace, and what is kernel private, but I wanted to
>> get feedback on the idea of requiring userspace to bracket access
>> first (vs. limiting this to coherent mappings or exporters who play
>> page faltings plus PTE shoot-down games) before I split the header
>> which would cause conflicts with other pending dma-buf patches.  So
>> flame-on!
>
> Why not just guard the kernel parts with __KERNEL__ or something? Or there
> are guidelines preventing this?

I think that is at least not how things are normally done, and seems a bit ugly

BR,
-R

>> [snip]
>>
>>
>> +
>> +static long dma_buf_ioctl(struct file *file, unsigned int cmd,
>> +               unsigned long arg)
>> +{
>> +       struct dma_buf *dmabuf;
>> +
>> +       if (!is_dma_buf_file(file))
>> +               return -EINVAL;
>> +
>> +       dmabuf = file->private_data;
>> +
>> +       switch (_IOC_NR(cmd)) {
>> +       case _IOC_NR(DMA_BUF_IOCTL_PREPARE_ACCESS):
>> +               if (dmabuf->ops->prepare_access)
>> +                       return dmabuf->ops->prepare_access(dmabuf);
>> +               return 0;
>> +       case _IOC_NR(DMA_BUF_IOCTL_FINISH_ACCESS):
>> +               if (dmabuf->ops->finish_access)
>> +                       return dmabuf->ops->finish_access(dmabuf);
>> +               return 0;
>> +       default:
>> +               return -EINVAL;
>> +       }
>> +}
>> +
>> +
>
> Multiple empty lines
>
>>  static int dma_buf_release(struct inode *inode, struct file *file)
>>  {
>>        struct dma_buf *dmabuf;
>> @@ -45,6 +85,8 @@ static int dma_buf_release(struct inode *inode, struct
>> file *file)
>>  }
>>
>>  static const struct file_operations dma_buf_fops = {
>> +       .mmap           = dma_buf_mmap,
>> +       .unlocked_ioctl = dma_buf_ioctl,
>>        .release        = dma_buf_release,
>>  };
>>
>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>> index a885b26..cbdff81 100644
>> --- a/include/linux/dma-buf.h
>> +++ b/include/linux/dma-buf.h
>> @@ -34,6 +34,17 @@
>>  struct dma_buf;
>>  struct dma_buf_attachment;
>>
>> +/* TODO: dma-buf.h should be the userspace visible header, and
>> dma-buf-priv.h (?)
>> + * the kernel internal header.. for now just stuff these here to avoid
>> conflicting
>> + * with other patches..
>> + *
>> + * For now, no arg to keep things simple, but we could consider adding an
>> + * optional region of interest later.
>> + */
>> +#define DMA_BUF_IOCTL_PREPARE_ACCESS   _IO('Z', 0)
>> +#define DMA_BUF_IOCTL_FINISH_ACCESS    _IO('Z', 1)
>> +
>> +
>
> Multiple empty lines
>
>>  /**
>>   * struct dma_buf_ops - operations possible on struct dma_buf
>>   * @attach: [optional] allows different devices to 'attach' themselves to
>> the
>> @@ -49,6 +60,13 @@ struct dma_buf_attachment;
>>   * @unmap_dma_buf: decreases usecount of buffer, might deallocate scatter
>>   *               pages.
>>   * @release: release this buffer; to be called after the last
>> dma_buf_put.
>> + * @mmap: [optional, allowed to fail] operation called if userspace calls
>> + *              mmap() on the dmabuf fd.  Note that userspace should use
>> the
>> + *              DMA_BUF_PREPARE_ACCESS / DMA_BUF_FINISH_ACCESS ioctls
>> before/after
>> + *              sw access to the buffer, to give the exporter an
>> opportunity to
>> + *              deal with cache maintenance.
>> + * @prepare_access: [optional] handler for PREPARE_ACCESS ioctl.
>> + * @finish_access: [optional] handler for FINISH_ACCESS ioctl.
>
> xx_access should only be optional if you don't implement mmap. Otherwise it
> will be very hard to implement cache sync in dma_buf (the cpu2dev and
> dev2cpu parts). Introducing cache sync in dma_buf should be a way to remove
> it from dma_buf clients. An option would be for the cache sync code to
> assume the worst for each cpu2dev sync. Even if the CPU has not touched
> anything.
>
> In short, very welcome patch ...
>
> /BR
> /Marcus
>
>
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