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

[Rob Clark <robdclark@gmail.com>]

On Wed, Feb 11, 2015 at 7:56 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> 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@arm.linux.org.uk> 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.

sure, my only point is that if I'm the odd man out, I can live with a
hack (ie. requiring drm/msm to be aware enough of the platform to know
if there is >1 level of address translation and frob'ing my 'struct
device' accordingly)... no point in a generic solution for one user.
I like to be practical.

BR,
-R

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch