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

[Petri Latvala <petri.latvala@intel.com>]

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.

> 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.

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.

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.


-- 
Petri Latvala
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