[PATCH] drm/amdgpu: Pin buffer while vmap'ing exported dma-buf objects

[Thomas Zimmermann]

Current dma-buf vmap semantics require that the mapped buffer remains
in place until the corresponding vunmap has completed.

For GEM-SHMEM, this used to be guaranteed by a pin operation while creating
an S/G table in import. GEM-SHMEN can now import dma-buf objects without
creating the S/G table, so the pin is missing. Leads to page-fault errors,
such as the one shown below.

[  102.101726] BUG: unable to handle page fault for address: ffffc90127000000
[...]
[  102.157102] RIP: 0010:udl_compress_hline16+0x219/0x940 [udl]
[...]
[  102.243250] Call Trace:
[  102.245695]  <TASK>
[  102.2477V95]  ? validate_chain+0x24e/0x5e0
[  102.251805]  ? __lock_acquire+0x568/0xae0
[  102.255807]  udl_render_hline+0x165/0x341 [udl]
[  102.260338]  ? __pfx_udl_render_hline+0x10/0x10 [udl]
[  102.265379]  ? local_clock_noinstr+0xb/0x100
[  102.269642]  ? __lock_release.isra.0+0x16c/0x2e0
[  102.274246]  ? mark_held_locks+0x40/0x70
[  102.278177]  udl_primary_plane_helper_atomic_update+0x43e/0x680 [udl]
[  102.284606]  ? __pfx_udl_primary_plane_helper_atomic_update+0x10/0x10 [udl]
[  102.291551]  ? lockdep_hardirqs_on_prepare.part.0+0x92/0x170
[  102.297208]  ? lockdep_hardirqs_on+0x88/0x130
[  102.301554]  ? _raw_spin_unlock_irq+0x24/0x50
[  102.305901]  ? wait_for_completion_timeout+0x2bb/0x3a0
[  102.311028]  ? drm_atomic_helper_calc_timestamping_constants+0x141/0x200
[  102.317714]  ? drm_atomic_helper_commit_planes+0x3b6/0x1030
[  102.323279]  drm_atomic_helper_commit_planes+0x3b6/0x1030
[  102.328664]  drm_atomic_helper_commit_tail+0x41/0xb0
[  102.333622]  commit_tail+0x204/0x330
[...]
[  102.529946] ---[ end trace 0000000000000000 ]---
[  102.651980] RIP: 0010:udl_compress_hline16+0x219/0x940 [udl]

In this stack strace, udl (based on GEM-SHMEM) imported and vmap'ed a
dma-buf from amdgpu. Amdgpu relocated the buffer, thereby invalidating the
mapping.

Provide a custom dma-buf vmap method in amdgpu that pins the object before
mapping it's buffer's pages into kernel address space. Do the opposite in
vunmap.

Note that dma-buf vmap differs from GEM vmap in how it handles relocation.
While dma-buf vmap keeps the buffer in place, GEM vmap requires the caller
to keep the buffer in place. Hence, this fix is in amdgpu's dma-buf code
instead of its GEM code.

A discussion of various approaches to solving the problem is available
at [1].

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
Fixes: 660cd44659a0 ("drm/shmem-helper: Import dmabuf without mapping its sg_table")
Reported-by: Thomas Zimmermann <tzimmermann at suse.de>
Closes: https://lore.kernel.org/dri-devel/ba1bdfb8-dbf7-4372-bdcb-df7e0511c702@suse.de/
Cc: Shixiong Ou <oushixiong at kylinos.cn>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Maxime Ripard <mripard at kernel.org>
Cc: David Airlie <airlied at gmail.com>
Cc: Simona Vetter <simona at ffwll.ch>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
Cc: "Christian König" <christian.koenig at amd.com>
Cc: dri-devel at lists.freedesktop.org
Cc: linux-media at vger.kernel.org
Cc: linaro-mm-sig at lists.linaro.org
Link: https://lore.kernel.org/dri-devel/9792c6c3-a2b8-4b2b-b5ba-fba19b153e21@suse.de/ # [1]
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 36 +++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
index 5743ebb2f1b7..5b33776eeece 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
@@ -285,6 +285,38 @@ static int amdgpu_dma_buf_begin_cpu_access(struct dma_buf *dma_buf,
 	return ret;
 }
 
+static int amdgpu_dma_buf_vmap(struct dma_buf *dma_buf, struct iosys_map *map)
+{
+	struct drm_gem_object *obj = dma_buf->priv;
+	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
+	int ret;
+
+	/*
+	 * Pin to keep buffer in place while it's vmap'ed. The actual
+	 * location is not important as long as it's mapable.
+	 *
+	 * This code is required for exporting to GEM-SHMEM without S/G table.
+	 * Once GEM-SHMEM supports dynamic imports, it should be dropped.
+	 */
+	ret = amdgpu_bo_pin(bo, AMDGPU_GEM_DOMAIN_MASK);
+	if (ret)
+		return ret;
+	ret = drm_gem_dmabuf_vmap(dma_buf, map);
+	if (ret)
+		amdgpu_bo_unpin(bo);
+
+	return ret;
+}
+
+static void amdgpu_dma_buf_vunmap(struct dma_buf *dma_buf, struct iosys_map *map)
+{
+	struct drm_gem_object *obj = dma_buf->priv;
+	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
+
+	drm_gem_dmabuf_vunmap(dma_buf, map);
+	amdgpu_bo_unpin(bo);
+}
+
 const struct dma_buf_ops amdgpu_dmabuf_ops = {
 	.attach = amdgpu_dma_buf_attach,
 	.pin = amdgpu_dma_buf_pin,
@@ -294,8 +326,8 @@ const struct dma_buf_ops amdgpu_dmabuf_ops = {
 	.release = drm_gem_dmabuf_release,
 	.begin_cpu_access = amdgpu_dma_buf_begin_cpu_access,
 	.mmap = drm_gem_dmabuf_mmap,
-	.vmap = drm_gem_dmabuf_vmap,
-	.vunmap = drm_gem_dmabuf_vunmap,
+	.vmap = amdgpu_dma_buf_vmap,
+	.vunmap = amdgpu_dma_buf_vunmap,
 };
 
 /**
-- 
2.50.1