Re: [RFC][PATCH 0/2] dma-buf: add importer private data for reimporting

From: êìì
Date: Tue Jun 04 2013 - 22:52:53 EST

On 2013ë 06ì 04ì 21:55, Daniel Vetter wrote:
> On Tue, Jun 04, 2013 at 07:42:22PM +0900, êìì wrote:
>> On 2013ë 06ì 01ì 00:29, Daniel Vetter wrote:
>>> On Fri, May 31, 2013 at 07:22:24PM +0900, êìì wrote:
>>>> Hello Daniel,
>>>> Thanks for your comment.
>>>> On 2013ë 05ì 31ì 18:14, Daniel Vetter wrote:
>>>>> On Fri, May 31, 2013 at 10:54 AM, Seung-Woo Kim <> wrote:
>>>>>> importer private data in dma-buf attachment can be used by importer to
>>>>>> reimport same dma-buf.
>>>>>> Seung-Woo Kim (2):
>>>>>> dma-buf: add importer private data to attachment
>>>>>> drm/prime: find gem object from the reimported dma-buf
>>>>> Self-import should already work (at least with the latest refcount
>>>>> fixes merged). At least the tests to check both re-import on the same
>>>>> drm fd and on a different all work as expected now.
>>>> Currently, prime works well for all case including self-importing,
>>>> importing, and reimporting as you describe. Just, importing dma-buf from
>>>> other driver twice with different drm_fd, each import create its own gem
>>>> object even two import is done for same buffer because prime_priv is in
>>>> struct drm_file. This means mapping to the device is done also twice.
>>>> IMHO, these duplicated creations and maps are not necessary if drm can
>>>> find previous import in different prime_priv.
>>> Well, that's imo a bug with the other driver. If it doesn't export
>>> something really simple (e.g. contiguous memory which doesn't require any
>>> mmio resources at all) it should have a cache of exported dma_buf fds so
>>> that it hands out the same dma_buf every time.
>> Hm, all existing dma-buf exporter including i915 driver implements its
>> map_dma_buf callback as allocating scatter-gather table with pages in
>> its buffer and calling dma_map_sg() with the sgt. With different
>> drm_fds, importing one dma-buf *twice*, then importer calls
>> dma_buf_attach() and dma_buf_map_attachment() twice at least in drm
>> importer because re-importing case can only checked with prime_priv in
>> drm_file as I described.
> Well, but thanks to all the self-import and re-import checks, it's
> _impossible_ to import the same dma_buf twice without noticing (presuming
> both importer and exporter are drm devices).

No, it is possible. Prime function, drm_gem_prime_fd_to_handle(), checks
re-import with following code.

ret = drm_prime_lookup_buf_handle(&file_priv->prime,
dma_buf, handle);

Unfortunately, file_priv is allocated per each open of drm node so this
code can only find re-import within same drm open context.

And driver specific import functions, like drm_gem_prime_import(), only
check self-import like following code.

if (dma_buf->ops == &drm_gem_prime_dmabuf_ops) {
obj = dma_buf->priv;
if (obj->dev == dev) {
/* ... */

This means some application like following can make re-import to
different gem objects.

int drm_fd1, drm_fd2, ret;
int dma_buf_fd;
struct drm_prime_handle prime1, prime2;

drm_fd1 = open(DRM_NODE, O_RDWR, 0);
drm_fd2 = open(DRM_NODE, O_RDWR, 0);

/* get some dma-buf_fd from other dma-buf exporter */
prime1.fd = dma_buf_fd;
prime2.fd = dma_buf_fd;

ret = ioctl(drm_fd1, DRM_IOCTL_PRIME_FD_TO_HANDLE, &prime1);
ret = ioctl(drm_fd2, DRM_IOCTL_PRIME_FD_TO_HANDLE, &prime2);

This will import same dma-buf twice as different GEM object because
above checking codes can not check already imported gem object from the

>>> Or it needs to be more clever in it's dma_buf_attachment_map functions and
>>> lookup up a pre-existing iommu mapping.
>>> But dealing with this in the importer is just broken.
>>>>> Second, the dma_buf_attachment is _definitely_ the wrong place to do
>>>>> this. If you need iommu mapping caching, that should happen at a lower
>>>>> level (i.e. in the map_attachment callback somewhere of the exporter,
>>>>> that's what the priv field in the attachment is for). Snatching away
>>>>> the attachement from some random other import is certainly not the way
>>>>> to go - attachements are _not_ refcounted!
>>>> Yes, attachments do not have refcount, so importer should handle and drm
>>>> case in my patch, importer private data is gem object and it has, of
>>>> course, refcount.
>>>> And at current, exporter can not classify map_dma_buf requests of same
>>>> importer to same buffer with different attachment because dma_buf_attach
>>>> always makes new attachments. To resolve this exporter should search all
>>>> different attachment from same importer of dma-buf and it seems more
>>>> complex than importer private data to me.
>>>> If I misunderstood something, please let me know.
>>> Like I've said above, just fix this in the exporter. If an importer sees
>>> two different dma_bufs it can very well presume that it those two indeed
>>> point to different backing storage.
>> Yes, my patch does not break this concept. I just fixed case importing
>> _one_ dma-buf twice with different drm_fds.
> See above, if you have two different struct file * for the same underlying
> buffer object something is wrong already.

drm_fds, I described, are not dma-buf fd but fds from opening DRM_NODE.

>>> This will be even more important if we attach fences two dma_bufs. If your
>>> broken exporter creates multiple dma_bufs each one of them will have their
>>> own fences attached, leading to a complete disasters. Ok, strictly
>>> speaking if you keep the same reservation pointer for each dma_buf it'll
>>> work, but that's just a detail of how you solve this in the exporter.
>> I can not understand about broken exporter you addressed. I don't mean
>> exporter makes dma-bufs from one backing storage.
>> While, my patch prevents not to create drm gem objects from one back
>> storage by importing one dma-buf with different drm-fds.
> Well, we also have code in drm prime for that case - if the same dma_buf
> object shows up multiple times, we'll only import it once. For the second
> import we'll return the already created drm_gem object from the first
> import, but with the refcount incremented.

I already describe import check code above and it can cause different
gem objects from one dma-buf even it works well to access real buffer.

>> I do not believe the fix of importer is the best way, but at this
>> moment, I have no idea how I can fix the exporter for this issue.
> I think if you have drm prime drivers both as importers and exporters, it
> is already fixed. It is correct though that both importer and exporter
> need a bit of code to take care and not accidentally duplicate a shared
> object somehow.

Dave's prime reference patch v6 fixes that junk prime handle remains in
&file_priv->prime after dma-buf is released and import/export
information is cleared to increase dma-buf f_count for each
&file_priv->prime. But issue I described still remains.

> But since you've proposed your rfc as part of the drm subsystem I've
> figured that we don't need to discuss the duplicate import handling code.

IMHO, considering current state of DRM PRIME, we need to discuss about
duplicate import handling. As I already wrote, I do not believe importer
private data is the best way to resolve this. So if you have better
solution, please let me know.

Thanks and Regards,
- Seung-Woo Kim

> Yours, Daniel

Seung-Woo Kim
Samsung Software R&D Center

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