Re: [PATCH 13/26] drm/i915: use the new iterator in i915_gem_busy_ioctl

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Am 20.09.21 um 12:33 schrieb Tvrtko Ursulin:
On 20/09/2021 11:13, Christian König wrote:
Am 20.09.21 um 10:45 schrieb Tvrtko Ursulin:

On 17/09/2021 13:35, Christian König wrote:
This makes the function much simpler since the complex
retry logic is now handled else where.

Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
  drivers/gpu/drm/i915/gem/i915_gem_busy.c | 32 ++++++++----------------
  1 file changed, 11 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_busy.c b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
index 6234e17259c1..b1cb7ba688da 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
@@ -82,8 +82,8 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
  {
      struct drm_i915_gem_busy *args = data;
      struct drm_i915_gem_object *obj;
-    struct dma_resv_list *list;
-    unsigned int seq;
+    struct dma_resv_iter cursor;
+    struct dma_fence *fence;
      int err;
        err = -ENOENT;
@@ -109,27 +109,17 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,        * to report the overall busyness. This is what the wait-ioctl does.
       *
       */
-retry:
-    seq = raw_read_seqcount(&obj->base.resv->seq);
-
-    /* Translate the exclusive fence to the READ *and* WRITE engine */ -    args->busy = busy_check_writer(dma_resv_excl_fence(obj->base.resv));
-
-    /* Translate shared fences to READ set of engines */
-    list = dma_resv_shared_list(obj->base.resv);
-    if (list) {
-        unsigned int shared_count = list->shared_count, i;
-
-        for (i = 0; i < shared_count; ++i) {
-            struct dma_fence *fence =
-                rcu_dereference(list->shared[i]);
-
+    args->busy = false;
+    dma_resv_iter_begin(&cursor, obj->base.resv, true);
+    dma_resv_for_each_fence_unlocked(&cursor, fence) {

You did not agree with my suggestion to reset args->busy on restart and so preserve current behaviour?

No, I want to keep the restart behavior internally to the dma_resv object and as far as I can see it should not make a difference here.

To be clear, on paper difference between old and new implementation is if the restart happens while processing the shared fences.

Old implementation unconditionally goes to "args->busy =
>>> busy_check_writer(dma_resv_excl_fence(obj->base.resv));" and so overwrites the set of flags returned to userspace.

New implementation can merge new read flags to the old set of flags and so return a composition of past and current fences.

Maybe it does not matter hugely in this case, depends if userspace typically just restarts until flags are clear. But I am not sure.

On the higher level - what do you mean with wanting to keep the restart behaviour internal? Not providing iterators users means of detecting it? I think it has to be provided.

Ok I will adjust that for now to get the patch set upstream. But in general when somebody outside of the dma_resv code base depends on the restart behavior then that's a bug inside the design of that code.

The callers should only care about what unsignaled fences are inside the dma_resv container and it shouldn't matter if those fences are presented once or multiple times because of a reset..

When this makes a difference we have a bug in the handling and should probably consider taking the dma_resv.lock instead.

Regards,
Christian.


Regards,

Tvrtko

Regards,
Christian.


Regards,

Tvrtko

+        if (dma_resv_iter_is_exclusive(&cursor))
+            /* Translate the exclusive fence to the READ *and* WRITE engine */
+            args->busy = busy_check_writer(fence);
+        else
+            /* Translate shared fences to READ set of engines */
              args->busy |= busy_check_reader(fence);
-        }
      }
-
-    if (args->busy && read_seqcount_retry(&obj->base.resv->seq, seq))
-        goto retry;
+    dma_resv_iter_end(&cursor);
        err = 0;
  out:






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