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@xxxxxxxxx> > >Signed-off-by: Sumit Semwal <sumit.semwal@xxxxxxxxxx> > > 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. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
