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authorChristopher James Halse Rogers <christopher.halse.rogers@canonical.com>2013-09-10 11:36:45 +0530
committerSumit Semwal <sumit.semwal@linaro.org>2013-09-10 11:36:45 +0530
commit19e8697ba45e7bcdb04f2adf6110fbf4882863e5 (patch)
tree249547a50a16d33e2c3c2dea96a592b2b823dc2b
parent9022e24e8946400d53719a761815069c3183e2bd (diff)
downloadop-kernel-dev-19e8697ba45e7bcdb04f2adf6110fbf4882863e5.zip
op-kernel-dev-19e8697ba45e7bcdb04f2adf6110fbf4882863e5.tar.gz
dma-buf: Expose buffer size to userspace (v2)
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>
-rw-r--r--Documentation/dma-buf-sharing.txt12
-rw-r--r--drivers/base/dma-buf.c29
2 files changed, 41 insertions, 0 deletions
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index e31a2a9..505e711 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -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
-------------------
diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
index 2d5ac1a..1e16cbd 100644
--- a/drivers/base/dma-buf.c
+++ b/drivers/base/dma-buf.c
@@ -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);
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