[RFC PATCH] drm/ttm: Allow the driver to resolve a WW transaction rollback

[Thomas Hellström]

Allow drivers to resolve a WW transaction rollback. This allows for
1) Putting a lower-priority transaction to sleep allowing another to
succeed instead both fighting using trylocks.
2) Letting the driver know whether a received -ENOMEM is the result of
competition with another WW transaction, which can be resolved using
rollback and retry or a real -ENOMEM which should be propagated back
to user-space as a failure.

Signed-off-by: Thomas Hellström <thomas.hellst...@linux.intel.com>
---
 drivers/gpu/drm/ttm/ttm_bo.c | 17 +++++++++++++++--
 include/drm/ttm/ttm_bo.h     |  2 ++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index bd5dae4d1624..c3ccbea2be3e 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -561,6 +561,10 @@ static int ttm_mem_evict_wait_busy(struct 
ttm_buffer_object *busy_bo,
        if (!busy_bo || !ticket)
                return -EBUSY;
 
+       /* We want to resolve contention before trying to lock again. */
+       if (ctx->propagate_edeadlk && ctx->contended_bo)
+               return  -EDEADLK;
+
        if (ctx->interruptible)
                r = dma_resv_lock_interruptible(busy_bo->base.resv,
                                                          ticket);
@@ -575,7 +579,15 @@ static int ttm_mem_evict_wait_busy(struct 
ttm_buffer_object *busy_bo,
        if (!r)
                dma_resv_unlock(busy_bo->base.resv);
 
-       return r == -EDEADLK ? -EBUSY : r;
+       if (r == -EDEADLK) {
+               if (ctx->propagate_edeadlk) {
+                       ttm_bo_get(busy_bo);
+                       ctx->contended_bo = busy_bo;
+               }
+               r = -EBUSY;
+       }
+
+       return r;
 }
 
 int ttm_mem_evict_first(struct ttm_device *bdev,
@@ -816,7 +828,7 @@ int ttm_bo_mem_space(struct ttm_buffer_object *bo,
                        goto error;
        }
 
-       ret = -ENOMEM;
+       ret = (ctx->propagate_edeadlk && ctx->contended_bo) ? -EDEADLK : 
-ENOMEM;
        if (!type_found) {
                pr_err(TTM_PFX "No compatible memory type found\n");
                ret = -EINVAL;
@@ -913,6 +925,7 @@ int ttm_bo_validate(struct ttm_buffer_object *bo,
                if (ret)
                        return ret;
        }
+
        return 0;
 }
 EXPORT_SYMBOL(ttm_bo_validate);
diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
index 8b113c384236..d8e35a794ce5 100644
--- a/include/drm/ttm/ttm_bo.h
+++ b/include/drm/ttm/ttm_bo.h
@@ -181,8 +181,10 @@ struct ttm_operation_ctx {
        bool gfp_retry_mayfail;
        bool allow_res_evict;
        bool force_alloc;
+       bool propagate_edeadlk;
        struct dma_resv *resv;
        uint64_t bytes_moved;
+       struct ttm_buffer_object *contended_bo;
 };
 
 /**
-- 
2.39.2