Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

From: Daniel Vetter
Date: Wed Feb 11 2015 - 07:55:56 EST

On Wed, Feb 11, 2015 at 06:23:52AM -0500, Rob Clark wrote:
> On Wed, Feb 11, 2015 at 6:12 AM, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxxx> wrote:
> > 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.
> to be completely generic, we would really need a way that the device
> could take over only just the last iommu (in case there were multiple
> levels of address translation)..

I still hold that if the dma api steals the iommu your gpu needs for
context switching then that's a bug in the platform setup code. dma api
really doesn't have any concept of switchable hw contexts. So trying to
work around this brokeness by mandating it as a valid dma-buf use-case is
totally backwards.
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 -
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