linux-kernel - [PATCH] drm/i915: Remove tautological compare in eb_relocate

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Date:   Sat, 23 Nov 2019 12:53:22 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH] drm/i915: Remove tautological compare in eb_relocate_vma

-Wtautological-compare was recently added to -Wall in LLVM, which
exposed an if statement in i915 that is always false:

../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
result of comparison of constant 576460752303423487 with expression of
type 'unsigned int' is always false
[-Wtautological-constant-out-of-range-compare]
        if (unlikely(remain > N_RELOC(ULONG_MAX)))
            ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

Since remain is an unsigned int, it can never be larger than UINT_MAX,
which is less than ULONG_MAX / sizeof(struct drm_i915_gem_relocation_entry).
Remove this statement to fix the warning.

Fixes: 2889caa92321 ("drm/i915: Eliminate lots of iterations over the execobjects array")
Link: https://github.com/ClangBuiltLinux/linux/issues/778
Link: https://github.com/llvm/llvm-project/commit/9740f9f0b6e5d7d5104027aee7edc9c5202dd052
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---

NOTE: Another possible fix for this is to change ULONG_MAX to UINT_MAX
      but I am not sure that is what was intended by this check. I'm
      happy to respin if that is the case.

 drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index f0998f1225af..9ed4379b4bc8 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -1482,8 +1482,6 @@ static int eb_relocate_vma(struct i915_execbuffer *eb, struct i915_vma *vma)
 
 	urelocs = u64_to_user_ptr(entry->relocs_ptr);
 	remain = entry->relocation_count;
-	if (unlikely(remain > N_RELOC(ULONG_MAX)))
-		return -EINVAL;
 
 	/*
 	 * We must check that the entire relocation array is safe
-- 
2.24.0