[PATCH 14/16] drm/i915: Remove locking for get_tiling

Chris Wilson
2016-08-01T18:23:08Z

Since we are not concerned with userspace racing itself with set-tiling
(the order is indeterminant even if we take a lock), then we can safely
read back the single obj->tiling_mode and do the static lookup of
swizzle mode without having to take a lock.

get-tiling is reasonably frequent due to the back-channel passing around
of tiling parameters in DRI2/DRI3.

v2: Make tiling_mode a full unsigned int so that we can trivially use it
with READ_ONCE(). Separating it out into manual control over the flags
field was too noisy for a simple patch. Note, that we can use the lower
bits of obj->stride for the tiling mode.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
drivers/gpu/drm/i915/i915_drv.h | 15 ++++++++-------
drivers/gpu/drm/i915/i915_gem_tiling.c | 10 +++-------
2 files changed, 11 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index d09c87fb207d..88b4fd8cb275 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2184,10 +2184,6 @@ struct drm_i915_gem_object {
unsigned int madv:2;

/**
- * Current tiling mode for the object.
- */
- unsigned int tiling_mode:2;
- /**
* Whether the tiling parameters for the currently associated fence
* register have changed. Note that for the purposes of tracking
* tiling changes we also treat the unfenced register, the register
@@ -2219,6 +2215,14 @@ struct drm_i915_gem_object {

atomic_t frontbuffer_bits;

+ /**
+ * Current tiling mode for the object.
+ */
+ unsigned int tiling_mode;
+
+ /** Current tiling stride for the object, if it's tiled. */
+ uint32_t stride;
+
unsigned int has_wc_mmap;
/** Count of VMA actually bound by this object */
unsigned int bind_count;
@@ -2246,9 +2250,6 @@ struct drm_i915_gem_object {
struct i915_gem_active last_write;
struct i915_gem_active last_fence;

- /** Current tiling stride for the object, if it's tiled. */
- uint32_t stride;
-
/** References from framebuffers, locks out tiling changes. */
unsigned long framebuffer_references;

diff --git a/drivers/gpu/drm/i915/i915_gem_tiling.c b/drivers/gpu/drm/i915/i915_gem_tiling.c
index b7f9875f69b4..c0e01333bddf 100644
--- a/drivers/gpu/drm/i915/i915_gem_tiling.c
+++ b/drivers/gpu/drm/i915/i915_gem_tiling.c
@@ -303,10 +303,8 @@ i915_gem_get_tiling(struct drm_device *dev, void *data,
if (!obj)
return -ENOENT;

- mutex_lock(&dev->struct_mutex);
-
- args->tiling_mode = obj->tiling_mode;
- switch (obj->tiling_mode) {
+ args->tiling_mode = READ_ONCE(obj->tiling_mode);
+ switch (args->tiling_mode) {
case I915_TILING_X:
args->swizzle_mode = dev_priv->mm.bit_6_swizzle_x;
break;
@@ -330,8 +328,6 @@ i915_gem_get_tiling(struct drm_device *dev, void *data,
if (args->swizzle_mode == I915_BIT_6_SWIZZLE_9_10_17)
args->swizzle_mode = I915_BIT_6_SWIZZLE_9_10;

- i915_gem_object_put(obj);
- mutex_unlock(&dev->struct_mutex);
-
+ i915_gem_object_put_unlocked(obj);
return 0;
}
--
2.8.1