Re: [PATCH 1/2] drm/i915/gem: Make our dma-buf exporter dynamic

On Wed, Jun 30, 2021 at 4:01 PM Daniel Vetter daniel@ffwll.ch wrote:

On Wed, Jun 30, 2021 at 03:07:00PM +0200, Thomas Hellström wrote:

...

If our exported dma-bufs are imported by another instance of our driver, that instance will typically have the imported dma-bufs locked during dma_buf_map_attachment(). But the exporter also locks the same reservation object in the map_dma_buf() callback, which leads to recursive locking.

Add a live selftest to exercise both dynamic and non-dynamic exports, and as a workaround until we fully support dynamic import and export, declare the exporter dynamic by providing pin() and unpin() implementations. For dynamic importers, make sure we keep the pinning also in map_dma_buf(), to ensure we never need to call dma_buf_move_notify(). Calling dma_buf_move_notify() is at the discretion of the exporter.

v2:

• Extend the selftest with a fake dynamic importer.
• Provide real pin and unpin callbacks to not abuse the interface.

Reported-by: Michael J. Ruhl michael.j.ruhl@intel.com Signed-off-by: Thomas Hellström thomas.hellstrom@linux.intel.com

I'm not happy with this, because i915 is currently violating the dma-resv fencing rules for dynamic dma-buf.

Yes since this is just the exporter we can probably get away with yolo'ing things, but Christian and me just spend a lot of angry typing figuring out what the rules actually are, so I really don't like bending them even more just because it's less typing.

To clarify what I meant here: I think the code is correct in the sense that it's not breaking any other existing code upstream in a functional or security relevant way.

What I meant with yolo merging is that if we land some dynamic dma-buf exporter support just to fix a bug which with slightly more lines can be fixed without resorting to quickly enabling dynamic dma-buf exporting while a) we know i915 is breaking dma-resv rules already and b) there was just a few weeks of rather angry discussions on this topic.

That's just a recipe to piss people off, at least if I'd be in Christian's shoes and see this land I'd get furious. So yolo on the collaboration and people side of things, not so much technically incorrect.

Plus with the sketch I described below we can fix the underlying issue we're seeing in a clean way, by essentially aligning what i915 does to what all other non-dynamic dma-buf ttm driver implementations do in drm_prime.c. Defacto that's the only way that works, and it is the contract for non-dynamic dma-buf for a driver using dma_resv_lock. The only reason we could get away without lockdep splats with our current dma-buf code in i915 of attempting to handle dma-buf more dynamic was because we used our completely independent locking design (and also never shared with another i915 instance). That illusion falls apart with i915 using dma-resv and with now multiple i915 instances being possible.

tldr; Using this way we can cleanly untangle solving the locking issue at hand from the fairly bigger topic of how we are going to support dynamic dma-buf and p2p and all that in i915.

I hope this explains a bit better why I have my take here like that. -Daniel

All we need for a quick interim fix is to not take the dma_resv_lock from our map/unamp callbacks. Pinning our backing storage from attach/detach callbacks (which are also called under dma_resv_lock) would also achieve that, without mudding any waters. So essentially just moving the pin/unpin_pages_unlocked and we should be good, which is almost as little typing.

Michael, since Thomas is on vacations now, care to type that up? The selftest is imo solid.

This is also consistent with what all other ttm based drivers do (aside from amdgpu, which is fully dynamic), see drm_gem_map_attach in drm_prime.c

Adding Christian as fyi. -Daniel

...

---

drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 31 ++++- .../drm/i915/gem/selftests/i915_gem_dmabuf.c | 116 +++++++++++++++++- 2 files changed, 143 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c index 616c3a2f1baf..918c19df7b66 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c @@ -12,6 +12,8 @@ #include "i915_gem_object.h" #include "i915_scatterlist.h"

+I915_SELFTEST_DECLARE(static bool force_different_devices;)

static struct drm_i915_gem_object *dma_buf_to_obj(struct dma_buf *buf) { return to_intel_bo(buf->priv); @@ -25,7 +27,14 @@ static struct sg_table *i915_gem_map_dma_buf(struct dma_buf_attachment *attachme struct scatterlist *src, *dst; int ret, i;

• ret = i915_gem_object_pin_pages_unlocked(obj);
  

• assert_object_held(obj);
  

• /*
  

•  * Note. In the dynamic importer case, the object is not yet pinned.
  

•  * Let's pin it here to avoid having to call the move_notify
  

•  * callback, The call of which is not yet implemented.
  

•  */
  

• ret = i915_gem_object_pin_pages(obj);
  if (ret)
          goto err;
  
  

@@ -168,6 +177,21 @@ static int i915_gem_end_cpu_access(struct dma_buf *dma_buf, enum dma_data_direct return err; }

+static int i915_gem_dmabuf_pin(struct dma_buf_attachment *attach) +{

• struct drm_i915_gem_object *obj = dma_buf_to_obj(attach->dmabuf);
  

• assert_object_held(obj);
  

• return i915_gem_object_pin_pages(obj);
  

+}

+static void i915_gem_dmabuf_unpin(struct dma_buf_attachment *attach) +{

• struct drm_i915_gem_object *obj = dma_buf_to_obj(attach->dmabuf);
  

• i915_gem_object_unpin_pages(obj);
  

+}

static const struct dma_buf_ops i915_dmabuf_ops = { .map_dma_buf = i915_gem_map_dma_buf, .unmap_dma_buf = i915_gem_unmap_dma_buf, @@ -177,6 +201,8 @@ static const struct dma_buf_ops i915_dmabuf_ops = { .vunmap = i915_gem_dmabuf_vunmap, .begin_cpu_access = i915_gem_begin_cpu_access, .end_cpu_access = i915_gem_end_cpu_access,

• .pin = i915_gem_dmabuf_pin,
  

• .unpin = i915_gem_dmabuf_unpin,
  

};

struct dma_buf *i915_gem_prime_export(struct drm_gem_object *gem_obj, int flags) @@ -241,7 +267,8 @@ struct drm_gem_object *i915_gem_prime_import(struct drm_device *dev, if (dma_buf->ops == &i915_dmabuf_ops) { obj = dma_buf_to_obj(dma_buf); /* is it from our device? */

•         if (obj->base.dev == dev) {
  

•         if (obj->base.dev == dev &&
  

•             !I915_SELFTEST_ONLY(force_different_devices)) {
                  /*
                   * Importing dmabuf exported from out own gem increases
                   * refcount on gem itself instead of f_count of dmabuf.
  

diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c index dd74bc09ec88..868b3469ecbd 100644 --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c @@ -35,7 +35,7 @@ static int igt_dmabuf_export(void *arg) static int igt_dmabuf_import_self(void *arg) { struct drm_i915_private *i915 = arg;

• struct drm_i915_gem_object *obj;
  

• struct drm_i915_gem_object *obj, *import_obj;
  struct drm_gem_object *import;
  struct dma_buf *dmabuf;
  int err;
  

@@ -65,14 +65,125 @@ static int igt_dmabuf_import_self(void *arg) err = -EINVAL; goto out_import; }

• import_obj = to_intel_bo(import);
  

• i915_gem_object_lock(import_obj, NULL);
  

• err = ____i915_gem_object_get_pages(import_obj);
  

• i915_gem_object_unlock(import_obj);
  

• if (err) {
  

•         pr_err("Same object dma-buf get_pages failed!\n");
  

•         goto out_import;
  

• }
  
  err = 0;
  

out_import:

• i915_gem_object_put(to_intel_bo(import));
  

• i915_gem_object_put(import_obj);
  

+out_dmabuf:

• dma_buf_put(dmabuf);
  

+out:

• i915_gem_object_put(obj);
  

• return err;
  

+}

+static void igt_dmabuf_move_notify(struct dma_buf_attachment *attach) +{

• GEM_WARN_ON(1);
  

+}

+static const struct dma_buf_attach_ops igt_dmabuf_attach_ops = {

• .move_notify = igt_dmabuf_move_notify,
  

+};

+static int igt_dmabuf_import_same_driver(void *arg) +{

• struct drm_i915_private *i915 = arg;
  

• struct drm_i915_gem_object *obj, *import_obj;
  

• struct drm_gem_object *import;
  

• struct dma_buf *dmabuf;
  

• struct dma_buf_attachment *import_attach;
  

• struct sg_table *st;
  

• long timeout;
  

• int err;
  

• force_different_devices = true;
  

• obj = i915_gem_object_create_shmem(i915, PAGE_SIZE);
  

• if (IS_ERR(obj))
  

•         goto out_ret;
  

• dmabuf = i915_gem_prime_export(&obj->base, 0);
  

• if (IS_ERR(dmabuf)) {
  

•         pr_err("i915_gem_prime_export failed with err=%d\n",
  

•                (int)PTR_ERR(dmabuf));
  

•         err = PTR_ERR(dmabuf);
  

•         goto out;
  

• }
  

• import = i915_gem_prime_import(&i915->drm, dmabuf);
  

• if (IS_ERR(import)) {
  

•         pr_err("i915_gem_prime_import failed with err=%d\n",
  

•                (int)PTR_ERR(import));
  

•         err = PTR_ERR(import);
  

•         goto out_dmabuf;
  

• }
  

• if (import == &obj->base) {
  

•         pr_err("i915_gem_prime_import reused gem object!\n");
  

•         err = -EINVAL;
  

•         goto out_import;
  

• }
  

• import_obj = to_intel_bo(import);
  

• i915_gem_object_lock(import_obj, NULL);
  

• err = ____i915_gem_object_get_pages(import_obj);
  

• if (err) {
  

•         pr_err("Different objects dma-buf get_pages failed!\n");
  

•         i915_gem_object_unlock(import_obj);
  

•         goto out_import;
  

• }
  

• /*
  

•  * If the exported object is not in system memory, something
  

•  * weird is going on. TODO: When p2p is supported, this is no
  

•  * longer considered weird.
  

•  */
  

• if (obj->mm.region != i915->mm.regions[INTEL_REGION_SMEM]) {
  

•         pr_err("Exported dma-buf is not in system memory\n");
  

•         err = -EINVAL;
  

• }
  

• i915_gem_object_unlock(import_obj);
  

• /* Now try a fake dynamic importer */
  

• import_attach = dma_buf_dynamic_attach(dmabuf, obj->base.dev->dev,
  

•                                        &igt_dmabuf_attach_ops,
  

•                                        NULL);
  

• if (IS_ERR(import_attach))
  

•         goto out_import;
  

• dma_resv_lock(dmabuf->resv, NULL);
  

• st = dma_buf_map_attachment(import_attach, DMA_BIDIRECTIONAL);
  

• dma_resv_unlock(dmabuf->resv);
  

• if (IS_ERR(st))
  

•         goto out_detach;
  

• timeout = dma_resv_wait_timeout(dmabuf->resv, false, true, 5 * HZ);
  

• if (!timeout) {
  

•         pr_err("dmabuf wait for exclusive fence timed out.\n");
  

•         timeout = -ETIME;
  

• }
  

• err = timeout > 0 ? 0 : timeout;
  

• dma_buf_unmap_attachment(import_attach, st, DMA_BIDIRECTIONAL);
  

+out_detach:

• dma_buf_detach(dmabuf, import_attach);
  

+out_import:

• i915_gem_object_put(import_obj);
  

out_dmabuf: dma_buf_put(dmabuf); out: i915_gem_object_put(obj); +out_ret:

• force_different_devices = false;
  return err;
  

}

@@ -286,6 +397,7 @@ int i915_gem_dmabuf_live_selftests(struct drm_i915_private *i915) { static const struct i915_subtest tests[] = { SUBTEST(igt_dmabuf_export),

•         SUBTEST(igt_dmabuf_import_same_driver),
  };
  
  return i915_subtests(tests, i915);
  

-- 2.31.1

-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch