Hi Christian
On 2/28/23 09:33, Christian König wrote:
This adds the infrastructure for an execution context for GEM buffers
which is similar to the existinc TTMs execbuf util and intended to replace
it in the long term.
The basic functionality is that we abstracts the necessary loop to lock
many different GEM buffers with automated deadlock and duplicate handling.
v2: drop xarray and use dynamic resized array instead, the locking
overhead is unecessary and measureable.
v3: drop duplicate tracking, radeon is really the only one needing that.
Signed-off-by: Christian König <christian.koenig@xxxxxxx>
Nice. This seems to have all we need for now for Xe as well, although
not for i915 ATM.
I see future exension of this for things like sleeping lock evictions
(where locks may go out-of-scope during the locking loop), including
other resv containers, passing through dma-buf move_notify() etc, what
are your thoughts around that? Extend this or build a different
functionality and make this a specialization of that?
What about the drm_gem_lock_reservations() interface? While a bit less
capable it's confusing having two apis doing essentially the same thing,
could we deprecate that?
Finally a comment below:
---
Documentation/gpu/drm-mm.rst | 12 ++
drivers/gpu/drm/Kconfig | 6 +
drivers/gpu/drm/Makefile | 2 +
drivers/gpu/drm/drm_exec.c | 249 +++++++++++++++++++++++++++++++++++
include/drm/drm_exec.h | 115 ++++++++++++++++
5 files changed, 384 insertions(+)
create mode 100644 drivers/gpu/drm/drm_exec.c
create mode 100644 include/drm/drm_exec.h
diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
index a79fd3549ff8..a52e6f4117d6 100644
--- a/Documentation/gpu/drm-mm.rst
+++ b/Documentation/gpu/drm-mm.rst
@@ -493,6 +493,18 @@ DRM Sync Objects
.. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
:export:
+DRM Execution context
+=====================
+
+.. kernel-doc:: drivers/gpu/drm/drm_exec.c
+ :doc: Overview
+
+.. kernel-doc:: include/drm/drm_exec.h
+ :internal:
+
+.. kernel-doc:: drivers/gpu/drm/drm_exec.c
+ :export:
+
GPU Scheduler
=============
diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 17d252dc25e2..84a5fc28c48d 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -200,6 +200,12 @@ config DRM_TTM
GPU memory types. Will be enabled automatically if a device driver
uses it.
+config DRM_EXEC
+ tristate
+ depends on DRM
+ help
+ Execution context for command submissions
+
config DRM_BUDDY
tristate
depends on DRM
diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index ab4460fcd63f..d40defbb0347 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -78,6 +78,8 @@ obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
#
# Memory-management helpers
#
+#
+obj-$(CONFIG_DRM_EXEC) += drm_exec.o
obj-$(CONFIG_DRM_BUDDY) += drm_buddy.o
diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
new file mode 100644
index 000000000000..df546cc5a227
--- /dev/null
+++ b/drivers/gpu/drm/drm_exec.c
@@ -0,0 +1,249 @@
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+
+#include <drm/drm_exec.h>
+#include <drm/drm_gem.h>
+#include <linux/dma-resv.h>
+
+/**
+ * DOC: Overview
+ *
+ * This component mainly abstracts the retry loop necessary for locking
+ * multiple GEM objects while preparing hardware operations (e.g. command
+ * submissions, page table updates etc..).
+ *
+ * If a contention is detected while locking a GEM object the cleanup procedure
+ * unlocks all previously locked GEM objects and locks the contended one first
+ * before locking any further objects.
+ *
+ * After an object is locked fences slots can optionally be reserved on the
+ * dma_resv object inside the GEM object.
+ *
+ * A typical usage pattern should look like this::
+ *
+ * struct drm_gem_object *obj;
+ * struct drm_exec exec;
+ * unsigned long index;
+ * int ret;
+ *
+ * drm_exec_init(&exec, true);
+ * drm_exec_while_not_all_locked(&exec) {
+ * ret = drm_exec_prepare_obj(&exec, boA, 1);
+ * drm_exec_continue_on_contention(&exec);
+ * if (ret)
+ * goto error;
+ *
+ * ret = drm_exec_lock(&exec, boB, 1);
Should be drm_exec_prepare_obj()?
+ * drm_exec_continue_on_contention(&exec);
+ * if (ret)
+ * goto error;
+ * }
+ *
+ * drm_exec_for_each_locked_object(&exec, index, obj) {
+ * dma_resv_add_fence(obj->resv, fence, DMA_RESV_USAGE_READ);
+ * ...
+ * }
+ * drm_exec_fini(&exec);
+ *
+ * See struct dma_exec for more details.
+ */
+
+/* Dummy value used to initially enter the retry loop */
+#define DRM_EXEC_DUMMY (void*)~0
+
+/* Unlock all objects and drop references */
+static void drm_exec_unlock_all(struct drm_exec *exec)
+{
+ struct drm_gem_object *obj;
+ unsigned long index;
+
+ drm_exec_for_each_locked_object(exec, index, obj) {
+ dma_resv_unlock(obj->resv);
+ drm_gem_object_put(obj);
+ }
+
+ if (exec->prelocked) {
+ dma_resv_unlock(exec->prelocked->resv);
+ drm_gem_object_put(exec->prelocked);
+ exec->prelocked = NULL;
+ }
+}
+
+/**
+ * drm_exec_init - initialize a drm_exec object
+ * @exec: the drm_exec object to initialize
+ * @interruptible: if locks should be acquired interruptible
+ *
+ * Initialize the object and make sure that we can track locked and duplicate
+ * objects.
+ */
+void drm_exec_init(struct drm_exec *exec, bool interruptible)
+{
+ exec->interruptible = interruptible;
+ exec->objects = kmalloc(PAGE_SIZE, GFP_KERNEL);
+
+ /* If allocation here fails, just delay that till the first use */
+ exec->max_objects = exec->objects ? PAGE_SIZE / sizeof(void *) : 0;
+ exec->num_objects = 0;
+ exec->contended = DRM_EXEC_DUMMY;
+ exec->prelocked = NULL;
+}
+EXPORT_SYMBOL(drm_exec_init);
+
+/**
+ * drm_exec_fini - finalize a drm_exec object
+ * @exec: the drm_exec object to finilize
+ *
+ * Unlock all locked objects, drop the references to objects and free all memory
+ * used for tracking the state.
+ */
+void drm_exec_fini(struct drm_exec *exec)
+{
+ drm_exec_unlock_all(exec);
+ kvfree(exec->objects);
+ if (exec->contended != DRM_EXEC_DUMMY) {
+ drm_gem_object_put(exec->contended);
+ ww_acquire_fini(&exec->ticket);
+ }
+}
+EXPORT_SYMBOL(drm_exec_fini);
+
+/**
+ * drm_exec_cleanup - cleanup when contention is detected
+ * @exec: the drm_exec object to cleanup
+ *
+ * Cleanup the current state and return true if we should stay inside the retry
+ * loop, false if there wasn't any contention detected and we can keep the
+ * objects locked.
+ */
+bool drm_exec_cleanup(struct drm_exec *exec)
+{
+ if (likely(!exec->contended)) {
+ ww_acquire_done(&exec->ticket);
+ return false;
+ }
+
+ if (likely(exec->contended == DRM_EXEC_DUMMY)) {
+ exec->contended = NULL;
+ ww_acquire_init(&exec->ticket, &reservation_ww_class);
+ return true;
+ }
+
+ drm_exec_unlock_all(exec);
+ exec->num_objects = 0;
+ return true;
+}
+EXPORT_SYMBOL(drm_exec_cleanup);
+
+/* Track the locked object in the xa and reserve fences */
+static int drm_exec_obj_locked(struct drm_exec *exec,
+ struct drm_gem_object *obj)
+{
+ if (unlikely(exec->num_objects == exec->max_objects)) {
+ size_t size = exec->max_objects * sizeof(void *);
+ void *tmp;
+
+ tmp = kvrealloc(exec->objects, size, size + PAGE_SIZE,
+ GFP_KERNEL);
+ if (!tmp)
+ return -ENOMEM;
+
+ exec->objects = tmp;
+ exec->max_objects += PAGE_SIZE / sizeof(void *);
+ }
+ drm_gem_object_get(obj);
+ exec->objects[exec->num_objects++] = obj;
+
+ return 0;
+}
+
+/* Make sure the contended object is locked first */
+static int drm_exec_lock_contended(struct drm_exec *exec)
+{
+ struct drm_gem_object *obj = exec->contended;
+ int ret;
+
+ if (likely(!obj))
+ return 0;
+
+ if (exec->interruptible) {
+ ret = dma_resv_lock_slow_interruptible(obj->resv,
+ &exec->ticket);
+ if (unlikely(ret))
+ goto error_dropref;
+ } else {
+ dma_resv_lock_slow(obj->resv, &exec->ticket);
+ }
+
+ ret = drm_exec_obj_locked(exec, obj);
+ if (unlikely(ret)) {
+ dma_resv_unlock(obj->resv);
+ goto error_dropref;
+ }
+
+ swap(exec->prelocked, obj);
+
+error_dropref:
+ /* Always cleanup the contention so that error handling can kick in */
+ drm_gem_object_put(obj);
+ exec->contended = NULL;
+ return ret;
+}
+
+/**
+ * drm_exec_prepare_obj - prepare a GEM object for use
+ * @exec: the drm_exec object with the state
+ * @obj: the GEM object to prepare
+ * @num_fences: how many fences to reserve
+ *
+ * Prepare a GEM object for use by locking it and reserving fence slots. All
+ * succesfully locked objects are put into the locked container. Duplicates
+ * detected as well and automatically moved into the duplicates container.
+ *
+ * Returns: -EDEADLK if a contention is detected, -ENOMEM when memory
+ * allocation failed and zero for success.
+ */
+int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
+ unsigned int num_fences)
+{
+ int ret;
+
+ ret = drm_exec_lock_contended(exec);
+ if (unlikely(ret))
+ return ret;
+
+ if (exec->prelocked == obj) {
+ drm_gem_object_put(exec->prelocked);
+ exec->prelocked = NULL;
+
+ return dma_resv_reserve_fences(obj->resv, num_fences);
+ }
+
+ if (exec->interruptible)
+ ret = dma_resv_lock_interruptible(obj->resv, &exec->ticket);
+ else
+ ret = dma_resv_lock(obj->resv, &exec->ticket);
+
+ if (unlikely(ret == -EDEADLK)) {
+ drm_gem_object_get(obj);
+ exec->contended = obj;
+ return -EDEADLK;
+ }
+
+ if (unlikely(ret))
+ return ret;
+
+ ret = drm_exec_obj_locked(exec, obj);
+ if (ret)
+ goto error_unlock;
+
+ /* Keep locked when reserving fences fails */
+ return dma_resv_reserve_fences(obj->resv, num_fences);
IMHO, keeping the object locked on a certain error code but not on
others is hard to follow both for humans and static analyzers and will
most certainly result in driver errors due to lacking -ENOMEM coverage.
In the unlikely -ENOMEM situation, could we have the caller handle that
explicitly by, for example, issue a new call with 0 fences reserved?
Thanks,
/Thomas
