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 <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Tested-by: Daniel Vetter <[email protected]>
---
Documentation/dma-buf-sharing.txt | 12 ++++++++++++
drivers/base/dma-buf.c | 28 ++++++++++++++++++++++++++++
2 files changed, 40 insertions(+)
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 0b23261..849e982 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 6687ba7..c33a857 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,
};
/*
@@ -133,6 +160,7 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops,
dmabuf->exp_name = exp_name;
file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, flags);
+ file->f_mode |= FMODE_LSEEK;
dmabuf->file = file;
--
1.8.3.2
Hi Christopher,
Le 04/09/2013 05:15, Christopher James Halse Rogers a ?crit :
> 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 <[email protected]>
> Reviewed-by: Daniel Vetter <[email protected]>
> Tested-by: Daniel Vetter <[email protected]>
> ---
> Documentation/dma-buf-sharing.txt | 12 ++++++++++++
> drivers/base/dma-buf.c | 28 ++++++++++++++++++++++++++++
> 2 files changed, 40 insertions(+)
>
> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> index 0b23261..849e982 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
Shouldn't it be "supported" instead of "support"?
Anyway, I'm just curious, in which case is it important to know the size?
Do we already have a way to get the dimensions (x, y and stripe)?
On Wed, Sep 04, 2013 at 12:24:27PM +0200, Martin Peres wrote:
> Hi Christopher,
> Le 04/09/2013 05:15, Christopher James Halse Rogers a ?crit :
> >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 <[email protected]>
> >Reviewed-by: Daniel Vetter <[email protected]>
> >Tested-by: Daniel Vetter <[email protected]>
> >---
> > Documentation/dma-buf-sharing.txt | 12 ++++++++++++
> > drivers/base/dma-buf.c | 28 ++++++++++++++++++++++++++++
> > 2 files changed, 40 insertions(+)
> >
> >diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> >index 0b23261..849e982 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
> Shouldn't it be "supported" instead of "support"?
>
> Anyway, I'm just curious, in which case is it important to know the size?
> Do we already have a way to get the dimensions (x, y and stripe)?
Size is an invariant part of a dma-buf. Everything else wrt metadata isn't
tracked in the kernel, so doesn't make much sense to expose it ;-)
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
Le 04/09/2013 14:05, Daniel Vetter a ?crit :
> On Wed, Sep 04, 2013 at 12:24:27PM +0200, Martin Peres wrote:
>> Hi Christopher,
>> Le 04/09/2013 05:15, Christopher James Halse Rogers a ?crit :
>>> 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 <[email protected]>
>>> Reviewed-by: Daniel Vetter <[email protected]>
>>> Tested-by: Daniel Vetter <[email protected]>
>>> ---
>>> Documentation/dma-buf-sharing.txt | 12 ++++++++++++
>>> drivers/base/dma-buf.c | 28 ++++++++++++++++++++++++++++
>>> 2 files changed, 40 insertions(+)
>>>
>>> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
>>> index 0b23261..849e982 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
>> Shouldn't it be "supported" instead of "support"?
>>
>> Anyway, I'm just curious, in which case is it important to know the size?
>> Do we already have a way to get the dimensions (x, y and stripe)?
> Size is an invariant part of a dma-buf. Everything else wrt metadata isn't
> tracked in the kernel, so doesn't make much sense to expose it ;-)
> -Daniel
ACK, thanks Daniel.