[PATCH] RFC: i915: Drop relocation support on Gen12+

[Jason Ekstrand]

The Vulkan driver in Mesa for Intel hardware never uses relocations if
it's running on a version of i915 that supports at least softpin which
all versions of i915 supporting Gen12 do.  On the OpenGL side, Gen12 is
only supported by iris which never uses relocations.  The older i965
driver in Mesa does use relocations but it only supports Intel hardware
through Gen11 and has been deprecated for all hardware Gen9+. The entire
relocation UAPI and related infrastructure, therefore, doesn't have any
open-source userspace consumer starting with Gen12.

Rejecting relocations starting with Gen12 has the benefit that we don't
have to bother supporting it on platforms with local memory.  Given how
much CPU touching of memory is required for relocations, not having to
do so on platforms where not all memory is directly CPU-accessible
carries significant advantages.

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Daniel Vetter <daniel.vetter at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 8 ++++++--
 1 file changed, 6 insertions(+), 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 4f9c1f5a4dedb..e10c93aff945d 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -1533,7 +1533,8 @@ eb_relocate_vma_slow(struct i915_execbuffer *eb, struct i915_vma *vma)
 	return err;
 }
 
-static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
+static int check_relocations(const struct i915_execbuffer *eb,
+			     const struct drm_i915_gem_exec_object2 *entry)
 {
 	const char __user *addr, *end;
 	unsigned long size;
@@ -1543,6 +1544,9 @@ static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
 	if (size == 0)
 		return 0;
 
+	if (size && eb->reloc_cache.gen >= 12)
+		return -EINVAL;
+
 	if (size > N_RELOC(ULONG_MAX))
 		return -EINVAL;
 
@@ -1576,7 +1580,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 		if (nreloc == 0)
 			continue;
 
-		err = check_relocations(&eb->exec[i]);
+		err = check_relocations(eb, &eb->exec[i]);
 		if (err)
 			goto err;
 
-- 
2.26.2