Re: [PATCH 35/39] drm/i915: Encode fence specific waitqueue behaviour into the wait.flags

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On 9/2/20 4:02 PM, Thomas Hellström (Intel) wrote:
Hi, Chris,

On 8/26/20 3:28 PM, Chris Wilson wrote:
Use the wait_queue_entry.flags to denote the special fence behaviour
(flattening continuations along fence chains, and for propagating
errors) rather than trying to detect ordinary waiters by their
functions.

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_sw_fence.c | 25 +++++++++++++++----------
  1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_sw_fence.c b/drivers/gpu/drm/i915/i915_sw_fence.c
index 4cd2038cbe35..4e557d1c4644 100644
--- a/drivers/gpu/drm/i915/i915_sw_fence.c
+++ b/drivers/gpu/drm/i915/i915_sw_fence.c
@@ -18,10 +18,15 @@
  #define I915_SW_FENCE_BUG_ON(expr) BUILD_BUG_ON_INVALID(expr)
  #endif
  -#define I915_SW_FENCE_FLAG_ALLOC BIT(3) /* after WQ_FLAG_* for safety */
-
  static DEFINE_SPINLOCK(i915_sw_fence_lock);
  +#define WQ_FLAG_BITS \
+    BITS_PER_TYPE(typeof_member(struct wait_queue_entry, flags))
+
+/* after WQ_FLAG_* for safety */
+#define I915_SW_FENCE_FLAG_FENCE BIT(WQ_FLAG_BITS - 1)
+#define I915_SW_FENCE_FLAG_ALLOC BIT(WQ_FLAG_BITS - 2)

Not sure if sharing the flags field with the wait.c implementation is fully OK either. Is the @key parameter to the wake function useable? I mean rather than passing just a list head could we pass something like

struct i915_sw_fence_key {
    bool no_recursion; /* Makes the wait function just put its entry on @continuation and return */
    int error;
    struct list_head continuation;
}

/Thomas


Actually, after doing some thinking, wouldn't just comparing against the internal wake function instead of the autoremove_wake_function remove the fragility. Would that be possible?

/Thomas


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