[PATCH 3/3] drm/amdgpu: add the BO-va mapping offset when kmapping an IB

[Junrui Luo via B4 Relay <devnull+moonafterrain.outlook.com@kernel.org>]

From: Junrui Luo <moonafterrain@outlook.com>

amdgpu_cs_patch_ibs() derives the CPU-side view of a UVD/VCE/VCN
indirect buffer from the BO returned by amdgpu_cs_find_mapping():

    r = amdgpu_bo_kmap(aobj, (void **)&kptr);
    kptr += va_start - (m->start * AMDGPU_GPU_PAGE_SIZE);

amdgpu_bo_kmap() returns the start of the BO, so only the displacement
of va_start inside the mapping is added.  The page tables, however, are
programmed from mapping->offset (see amdgpu_vm_bo_update()), which
records the offset_in_bo the client passed to AMDGPU_GEM_VA.  The GPU
therefore resolves va_start to BO byte

    m->offset + (va_start - m->start * AMDGPU_GPU_PAGE_SIZE)

while the kernel inspects the byte m->offset lower.  Whenever an IB is
submitted through a mapping created with a non-zero offset_in_bo, the
two views disagree.

Add the missing term so the kmapped pointer describes the same bytes the
page tables do.

Every other CPU-side consumer of amdgpu_cs_find_mapping() omits
mapping->offset in the same way.

Fixes: 4802ce117786 ("drm/amdgpu: fix UVD/VCE VM emulation")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
index 9c514cb01096..a72cee871af3 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
@@ -1047,7 +1047,7 @@ static int amdgpu_cs_patch_ibs(struct amdgpu_cs_parser *p,
 		if (r)
 			return r;
 
-		kptr += va_start - (m->start * AMDGPU_GPU_PAGE_SIZE);
+		kptr += m->offset + va_start - (m->start * AMDGPU_GPU_PAGE_SIZE);
 
 		if (ring->funcs->parse_cs) {
 			memcpy(ib->ptr, kptr, ib->length_dw * 4);

-- 
2.51.2