[igt-dev] [PATCH i-g-t 1/7] lib/i915/gem_mman: add FIXED mmap mode

[Dixit, Ashutosh]

On Tue, 27 Jul 2021 19:01:24 -0700, Dixit, Ashutosh wrote:
>
> On Mon, 26 Jul 2021 05:03:04 -0700, Matthew Auld wrote:
> >
> > diff --git a/lib/i915/gem_mman.c b/lib/i915/gem_mman.c
> > index 4b4f2114..e2514f0c 100644
> > --- a/lib/i915/gem_mman.c
> > +++ b/lib/i915/gem_mman.c
> > @@ -497,6 +497,43 @@ void *gem_mmap_offset__cpu(int fd, uint32_t handle, uint64_t offset,
> >	return ptr;
> >  }
> >
> > +#define LOCAL_I915_MMAP_OFFSET_FIXED 4
>
> Cc: @Petri Latvala
>
> This use of LOCAL declarations is more related to the methodology we follow
> in IGT rather than this patch. We have seen in the past that such LOCAL's
> linger on in the code for months and years till someone decides to clean
> them so the question is can we prevent these LOCAL's from getting
> introduced in the first place.
>
> One reason for these is that we sync IGT headers with drm-next whereas IGT
> is used to test drm-tip. So the delta between the two results in such
> LOCAL's as in this case.
>
> My proposal is that even if we don't start sync'ing IGT headers with
> drm-tip (instead of drm-next) we allow direct modification of the headers
> when needed to avoid introducing such LOCAL's. So in the above case we
> would add:
>
> #define I915_MMAP_OFFSET_FIXED 4
>
> to i915_drm.h as part of this patch and then just use
> I915_MMAP_OFFSET_FIXED. If another sync happens from drm-next before this
> #define has appeared there, the compile will break and whoever syncs will
> need to add this again to i915_drm.h.
>
> What do people think about a scheme such as this? The other, perhaps
> better, option of course is to sync the headers directly with drm-tip
> (whenever and as often as needed). But the goal in both cases is to avoid
> LOCAL's, or other things like #ifndef's distributed throughout multiple
> source files which we also do in such cases. A centralized internal header
> to contain such declarations might not be so bad. Thanks.

I have submitted a patch based on this last statement here:

https://patchwork.freedesktop.org/series/93096/