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dma-buf: Expose buffer size to userspace (v2)

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Each dma-buf has an associated size and it's reasonable for userspace
to want to know what it is.

Since userspace already has an fd, expose the size using the
size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
idiom.

v2: Added Daniel's sugeested documentation, with minor fixups

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
This commit is contained in:
Christopher James Halse Rogers 2013-09-10 11:36:45 +05:30 committed by Sumit Semwal
parent 9022e24e89
commit 19e8697ba4
2 changed files with 41 additions and 0 deletions
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12
Documentation/dma-buf-sharing.txt
View File

@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
interesting ways depending upong the exporter (if userspace starts depending
upon this implicit synchronization).
Other Interfaces Exposed to Userspace on the dma-buf FD
------------------------------------------------------
- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
llseek operation will report -EINVAL.
If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
cases. Userspace can use this to detect support for discovering the dma-buf
size using llseek.
Miscellaneous notes
-------------------

29
drivers/base/dma-buf.c
View File

@ -77,9 +77,36 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
return dmabuf->ops->mmap(dmabuf, vma);
}
static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
{
struct dma_buf *dmabuf;
loff_t base;
if (!is_dma_buf_file(file))
return -EBADF;
dmabuf = file->private_data;
/* only support discovering the end of the buffer,
but also allow SEEK_SET to maintain the idiomatic
SEEK_END(0), SEEK_CUR(0) pattern */
if (whence == SEEK_END)
base = dmabuf->size;
else if (whence == SEEK_SET)
base = 0;
else
return -EINVAL;
if (offset != 0)
return -EINVAL;
return base + offset;
}
static const struct file_operations dma_buf_fops = {
.release = dma_buf_release,
.mmap = dma_buf_mmap_internal,
.llseek = dma_buf_llseek,
};
/*
@ -137,6 +164,8 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops,
kfree(dmabuf);
return ERR_CAST(file);
}
file->f_mode |= FMODE_LSEEK;
dmabuf->file = file;
mutex_init(&dmabuf->lock);