[RFC 1/3] drm/amdgpu: Reject impossible entities early

[Tvrtko Ursulin <tvrtko.ursulin@igalia.com>]

Currently there are two different behaviour modes when userspace tries to
operate on not present HW IP blocks. On a machine without UVD, VCE and VPE
blocks, this can be observed for example like this:

$ sudo ./amd_fuzzing --r cs-wait-fuzzing
...
amd_cs_wait_fuzzing DRM_IOCTL_AMDGPU_CTX r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_GFX r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_COMPUTE r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_DMA r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_UVD r -1
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCE r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_UVD_ENC r -1
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCN_DEC r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCN_ENC r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCN_JPEG r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VPE r 0

We can see that UVD returns an errno (-EINVAL) from the CS_WAIT ioctl,
while VCE and VPE return unexpected successes.

The difference stems from the fact the UVD is a load balancing engine
which retains the context, so with a workaround implemented in
amdgpu_ctx_init_entity(), but which does not account for the fact hardware
block may not be present.

This causes a single NULL scheduler to be passed to
drm_sched_entity_init(), which immediately rejects this with -EINVAL.

The not present VCE and VPE cases on the other hand pass zero schedulers
to drm_sched_entity_init(), which is explicitly allowed and results in
unusable entities.

As the UVD case however shows, call paths can handle the errors, so we can
consolidate this into a single path which will always return -EINVAL if
the HW IP block is not present.

We do this by rejecting it early and not calling drm_sched_entity_init()
when there is no backing hardware.

This also removes the need for the drm_sched_entity_init() to handle the
zero schedulers and NULL scheduler cases, which means that we can follow
up by removing the special casing from the DRM scheduler.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
References: f34e8bb7d6c6 ("drm/sched: fix null-ptr-deref in init entity")
Cc: Christian König <christian.koenig@amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
index afedea02188d..a5f85ea9fbb6 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
@@ -239,6 +239,11 @@ static int amdgpu_ctx_init_entity(struct amdgpu_ctx *ctx, u32 hw_ip,
 			goto error_free_entity;
 	}
 
+	if (num_scheds == 0) {
+		r = -EINVAL;
+		goto error_free_entity;
+	}
+
 	/* disable load balance if the hw engine retains context among dependent jobs */
 	if (hw_ip == AMDGPU_HW_IP_VCN_ENC ||
 	    hw_ip == AMDGPU_HW_IP_VCN_DEC ||
-- 
2.52.0