On Thu, Jan 29, 2015 at 10:47 AM, Russell King - ARM Linux linux@arm.linux.org.uk wrote:
On Thu, Jan 29, 2015 at 09:00:11PM +0530, Sumit Semwal wrote:
So, short answer is, it is left to the exporter to decide. The dma-buf framework should not even attempt to decide or enforce any of the above.
At each dma_buf_attach(), there's a callback to the exporter, where the exporter can decide, if it intends to handle these kind of cases, on the best way forward.
The exporter might, for example, decide to migrate backing storage,
That's a decision which the exporter can not take. Think about it...
If subsystem Y has mapped the buffer, it could be accessing the buffer's backing storage at the same time that subsystem Z tries to attach to the buffer.
The *theory* is that Y is map/unmap'ing the buffer around each use, so there will be some point where things could be migrated and remapped.. in practice, I am not sure that anyone is doing this yet.
Probably it would be reasonable if a more restrictive subsystem tried to attach after the buffer was already allocated and mapped in a way that don't meet the new constraints, then -EBUSY.
But from a quick look it seems like there needs to be a slight fixup to not return 0 if calc_constraints() fails..
Once the buffer has been exported to another user, the exporter has effectively lost control over mediating accesses to that buffer.
All that it can do with the way the dma-buf API is today is to allocate a _different_ scatter list pointing at the same backing storage which satisfies the segment size and number of segments, etc.
There's also another issue which you haven't addressed. What if several attachments result in lowering max_segment_size and max_segment_count such that:
max_segment_size * max_segment_count < dmabuf->sizebut individually, the attachments allow dmabuf->size to be represented as a scatterlist?
Quite possibly for some of these edge some of cases, some of the dma-buf exporters are going to need to get more clever (ie. hand off different scatterlists to different clients). Although I think by far the two common cases will be "I can support anything via an iommu/mmu" and "I need phys contig".
But that isn't an issue w/ dma-buf itself, so much as it is an issue w/ drivers. I guess there would be more interest in fixing up drivers when actual hw comes along that needs it..
BR, -R
If an exporter were to take notice of the max_segment_size and max_segment_count, the resulting buffer is basically unrepresentable as a scatterlist.
Please consider the possible sequences of use (such as the scenario above) when creating or augmenting an API.
I tried to think of the scenarios I could think of, but If you still feel this approach doesn't help with your concerns, I'll graciously accept advice to improve it.
See the new one above :)
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