Re: [RFC PATCH i-g-t v4 2/4] lib: Add minimum GTT alignment helper

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Janusz Krzysztofik (2019-10-31 15:28:55)
> Some tests assume 4kB offset alignment while using softpin.  That
> assumption may be wrong on future GEM backends with possibly larger
> minimum page sizes.  As a result, those tests may either fail on
> softpin at offsets which are incorrectly aligned, may silently skip
> such incorrectly aligned addresses assuming them occupied by other
> users if incorrect detection method is used, or may always succeed
> when examining invalid use patterns.
> 
> Provide a helper function that detects minimum GTT alignment.  Tests
> may use it to calculate softpin offsets valid for actually used backing
> store.
> 
> v2: Rename the helper, use 'minimum GTT alignment' term across the
>     change (Chris),
>   - use error numbers to distinguish between invalid offsets and
>     addresses occupied by other users, then
>   - simplify the code (Chris).
> 
> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> Cc: Stuart Summers <stuart.summers@intel.com>
> ---
>  lib/ioctl_wrappers.c | 46 ++++++++++++++++++++++++++++++++++++++++++++
>  lib/ioctl_wrappers.h |  2 ++
>  2 files changed, 48 insertions(+)
> 
> diff --git a/lib/ioctl_wrappers.c b/lib/ioctl_wrappers.c
> index 628f8b83..f0ef8b2e 100644
> --- a/lib/ioctl_wrappers.c
> +++ b/lib/ioctl_wrappers.c
> @@ -54,6 +54,7 @@
>  #include "intel_io.h"
>  #include "igt_debugfs.h"
>  #include "igt_sysfs.h"
> +#include "igt_x86.h"
>  #include "config.h"
>  
>  #ifdef HAVE_VALGRIND
> @@ -1158,6 +1159,51 @@ bool gem_has_softpin(int fd)
>         return has_softpin;
>  }
>  
> +/**
> + * gem_gtt_min_alignment_order:
> + * @fd: open i915 drm file descriptor
> + *
> + * This function detects the minimum possible alignment of a soft-pinned gem
> + * object allocated from a default backing store.  It is useful for calculating
> + * correctly aligned softpin offsets.
> + * Since size order to size conversion (size = 1 << order) is less trivial
> + * than the opposite, the function returns the alignment order as more handy.
> + *
> + * Returns:
> + * Size order of the minimum GTT alignment
> + */
> +int gem_gtt_min_alignment_order(int fd)

But not part of ioctl_wrappers.c!

lib/i915/gem_gtt_topology.c? _query.c? _probe.c?

> +{
> +       struct drm_i915_gem_exec_object2 obj;
> +       struct drm_i915_gem_execbuffer2 eb;
> +       const uint32_t bbe = MI_BATCH_BUFFER_END;
> +       int order;
> +
> +       /* no softpin => 4kB page size */
> +       if (!gem_has_softpin(fd))
> +               return 12;
> +
> +       memset(&obj, 0, sizeof(obj));
> +       memset(&eb, 0, sizeof(eb));
> +
> +       obj.handle = gem_create(fd, 4096);
> +       obj.flags = EXEC_OBJECT_PINNED | EXEC_OBJECT_SUPPORTS_48B_ADDRESS;
> +       eb.buffers_ptr = to_user_pointer(&obj);
> +       eb.buffer_count = 1;
> +       gem_write(fd, obj.handle, 0, &bbe, sizeof(bbe));
> +
> +       for (order = 12; order < 64; order++) {
> +               obj.offset = 1ull << order;
> +               if (__gem_execbuf(fd, &eb) != -EINVAL)
> +                       break;
> +       }
> +       igt_assert(obj.offset < gem_aperture_size(fd));

Hmm, there is one more trick we can apply here: an invalid reloc.

memset(&reloc, 0, sizeof(reloc));
obj.relocs_ptr = to_user_pointer(&reloc);
obj.relocation_count = 1;

We first process the buffers, then we process the relocations. So we
will get the -EINVAL for illegal softpin before we get the -ENOENT for
invalid reloc handle.

That just saves actually emitting a request.
-Chris
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