On Wed, Feb 11, 2015 at 09:28:37AM +0100, Marek Szyprowski wrote:
Hello,
On 2015-01-27 09:25, Sumit Semwal wrote:
Add some helpers to share the constraints of devices while attaching to the dmabuf buffer.
At each attach, the constraints are calculated based on the following:
- max_segment_size, max_segment_count, segment_boundary_mask from device_dma_parameters.
In case the attaching device's constraints don't match up, attach() fails.
At detach, the constraints are recalculated based on the remaining attached devices.
Two helpers are added:
- dma_buf_get_constraints - which gives the current constraints as calculated during each attach on the buffer till the time,
- dma_buf_recalc_constraints - which recalculates the constraints for all currently attached devices for the 'paranoid' ones amongst us.
The idea of this patch is largely taken from Rob Clark's RFC at https://lkml.org/lkml/2012/7/19/285, and the comments received on it.
Cc: Rob Clark robdclark@gmail.com Signed-off-by: Sumit Semwal sumit.semwal@linaro.org
The code looks okay, although it will probably will work well only with typical cases like 'contiguous memory needed' or 'no constraints at all' (iommu).
Which is a damn good reason to NAK it - by that admission, it's a half-baked idea.
If all we want to know is whether the importer can accept only contiguous memory or not, make a flag to do that, and allow the exporter to test this flag. Don't over-engineer this to make it _seem_ like it can do something that it actually totally fails with.
As I've already pointed out, there's a major problem if you have already had a less restrictive attachment which has an active mapping, and a new more restrictive attachment comes along later.
It seems from Rob's descriptions that we also need another flag in the importer to indicate whether it wants to have a valid struct page in the scatter list, or whether it (correctly) uses the DMA accessors on the scatter list - so that exporters can reject importers which are buggy.