drm/i915: simplify shmem pwrite/pread slowpath handling

The shmem paths for pwrite/pread used a clever trick to hold onto a
single page when dropping the big dev->struct_mutex for the slowpath.
But this ran the risk of reinstating (or not completely purging) the
backing storage when dropping purgeable objects.

Hence the code needed to keep track of whether it ever dropped the
lock, and if it did, manually check whether it needs to re-purge the
backing storage. But thanks to the pages pin count introduced in

commit a5570178
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Sep 4 21:02:54 2012 +0100

    drm/i915: Pin backing pages whilst exporting through a dmabuf vmap

which allowed us to pin the backing storage and remove that page
reference trick from shmem_pwrite/read in

commit f60d7f0c
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Sep 4 21:02:56 2012 +0100

    drm/i915: Pin backing pages for pread

and

commit 755d2218
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Sep 4 21:02:55 2012 +0100

    drm/i915: Pin backing pages for pwrite

we can now abolish this check. The slowpath cleanup completely
disappears from pread, and for pwrite we're only left with the domain
fixup in case someone moved the object out of the cpu domain from
under us. A follow-on patch will optimize that a notch more.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -407,7 +407,6 @@ i915_gem_shmem_pread(struct drm_device *dev,
 	loff_t offset;
 	int shmem_page_offset, page_length, ret = 0;
 	int obj_do_bit17_swizzling, page_do_bit17_swizzling;
-	int hit_slowpath = 0;
 	int prefaulted = 0;
 	int needs_clflush = 0;
 	struct scatterlist *sg;
@@ -469,7 +468,6 @@ i915_gem_shmem_pread(struct drm_device *dev,
 		if (ret == 0)
 			goto next_page;
 
-		hit_slowpath = 1;
 		mutex_unlock(&dev->struct_mutex);
 
 		if (!prefaulted) {
@@ -502,12 +500,6 @@ i915_gem_shmem_pread(struct drm_device *dev,
 out:
 	i915_gem_object_unpin_pages(obj);
 
-	if (hit_slowpath) {
-		/* Fixup: Kill any reinstated backing storage pages */
-		if (obj->madv == __I915_MADV_PURGED)
-			i915_gem_object_truncate(obj);
-	}
-
 	return ret;
 }
 
@@ -838,11 +830,8 @@ i915_gem_shmem_pwrite(struct drm_device *dev,
 	i915_gem_object_unpin_pages(obj);
 
 	if (hit_slowpath) {
-		/* Fixup: Kill any reinstated backing storage pages */
-		if (obj->madv == __I915_MADV_PURGED)
-			i915_gem_object_truncate(obj);
-		/* and flush dirty cachelines in case the object isn't in the cpu write
-		 * domain anymore. */
+		/* Fixup: Flush dirty cachelines in case the object isn't in the
+		 * cpu write domain anymore. */
 		if (obj->base.write_domain != I915_GEM_DOMAIN_CPU) {
 			i915_gem_clflush_object(obj);
 			i915_gem_chipset_flush(dev);