[Intel-gfx] [PATCH 11/12] drm/i915: Extend GET_APERTURE ioctl to report available map space

[Chris Wilson]

On Fri, Apr 29, 2016 at 11:26:45AM +0100, Tvrtko Ursulin wrote:
> 
> On 29/04/16 11:18, Chris Wilson wrote:
> >On Fri, Apr 29, 2016 at 11:06:49AM +0100, Tvrtko Ursulin wrote:
> >>I don't get it - if we are adding something why not add it in a way
> >>that makes it clear and self-contained - what is the downside of
> >>what I propose to meet such resistance?
> >
> >You're suggesting to add a field I'm not going to use. Is any one?
> >Until there is an actual use (which would afaict be if one of the
> >existing values changed meaning, which itself would be an abi break) I'm
> >not convinced we should be designing for the unknowable. If we did need
> >to version would we not be better off with a new ioctl?
> 
> I am suggesting if we are adding aper_total_size, or whatever it is
> called, to make it usable in an self-contained matter.
> 
> My impression was you want aper_total_size so I was simply objecting
> to adding it without being able to directly tell if kernel actually
> supports that extension.

The two additions that I thought we have reduced it to:

	u64 mappable_aperture_size
	u64 stolen_size;

Of which I agree that the first has some ambiguity over 0. But I don't
think it makes a difference in practice. I for one would not bother with
checking a version. I already have a cascade here to deal with not being
able to use various probes, with an eventual fallback to a safe value.

We know the mappable_aperture_size cannot be zero with the exception
that the device is disabled - but we have opened the device already.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre