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

[Dixit, Ashutosh]

On Tue, 27 Jul 2021 23:08:40 -0700, Petri Latvala wrote:
>
> On Tue, Jul 27, 2021 at 07:01:24PM -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.
>
> I don't like that kind of a breakage at all. That enforces mandatory
> fixups to some poor developer working on unrelated code who doesn't
> necessarily know how to even fix it easily.
>
> Of course an argument can be made that it's an i915 token in an i915
> header so it will be the i915 people doing it, but for a general case
> that's going to cause more harm than it solves problems, I feel.

OK, let's not change anything with the headers we import for now.

> > 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.
>
> A separate manually written header for new tokens that are not yet in
> drm-next might be the least bad of all options. Although now that I've
> said it, the perfect world would have new tokens done like this:
>
> #ifndef I915_MMAP_OFFSET_FIXED
> #define I915_MMAP_OFFSET_FIXED 4
> #else
> _Static_assert(I915_MMAP_OFFSET_FIXED == 4, "ABI broken, yikes");
> #endif
>
> In a different language wrapping all that in a
>
> MAYBE_DECLARE(I915_MMAP_OFFSET_FIXED, 4)
>
> might be easier but with C preprocessor it's a bit more... involved. A
> separate build-time script to generate that header maybe? Such a
> script could also just completely omit the definition if header copies
> already introduce the token.

IMO all this wouldn't do much more that what gcc already does. For example,
for this:

#define I915_MMAP_OFFSET_FIXED 4
#define I915_MMAP_OFFSET_FIXED 4

gcc silently ignores the second #define, whereas for:

#define I915_MMAP_OFFSET_FIXED 4
#define I915_MMAP_OFFSET_FIXED 5

gcc will warn that second I915_MMAP_OFFSET_FIXED is redefined.

And gcc will error out on things like struct redeclaration.

> Recap:
>
> 1) We have kernel headers copied into IGT to ensure it builds fine
> without latest-and-greatest headers installed on the system.
>
> 2) Copies are from drm-next to ensure the next person to copy the
> headers doesn't accidentally drop definitions that originate from a
> vendor-specific tree. (That same reason is also for why one shouldn't
> edit the headers manually)
>
> 3) To get to drm-next, the kernel code needs to be tested with IGT
> first, so we need new definitions to test that kernel code in some
> form.

I guess it is possible to test with "Test-with:" and merge the kernel
changes first and the IGT changes later with the correct headers but maybe
it's inconvenient? But don't we merge the kernel changes before IGT?

> 4) LOCAL_* definitions that are cleaned up later when actual
> definitions reach drm-next sounds good in theory but in practice the
> cleaning part is often forgotten.
>
> Either way, I think the code using new definitions should use the
> intended final names so we should just entirely drop the practice of
> declaring anything LOCAL_*. That way the cleanup is limited to one
> place.

In any case, any thoughts about the i915_drm_local.h patch I posted:

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

I am not asking for any other changes to anything at this this point. I
have also started asking people to not use the LOCAL_ or local_ prefix any
more in code reviews. But I probably still prefer that these declarations
move to a central place such as i915_drm_local.h if possible so it's easier
to clean them up later. Thanks.