On Wed, Feb 11, 2015 at 09:28:37AM +0100, Marek Szyprowski wrote:

Hello,

On 2015-01-27 09:25, Sumit Semwal wrote:

quoted

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 <redacted>
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.

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