[PATCH v4 21/33] drm/msm: Add _NO_SHARE flag
Christian König
christian.koenig at amd.com
Mon May 5 15:17:18 UTC 2025
On 5/5/25 16:15, Rob Clark wrote:
> On Mon, May 5, 2025 at 12:54 AM Christian König
> <christian.koenig at amd.com> wrote:
>>
>> On 5/2/25 18:56, Rob Clark wrote:
>>> From: Rob Clark <robdclark at chromium.org>
>>>
>>> Buffers that are not shared between contexts can share a single resv
>>> object. This way drm_gpuvm will not track them as external objects, and
>>> submit-time validating overhead will be O(1) for all N non-shared BOs,
>>> instead of O(n).
>>>
>>> Signed-off-by: Rob Clark <robdclark at chromium.org>
>>> ---
>>> drivers/gpu/drm/msm/msm_drv.h | 1 +
>>> drivers/gpu/drm/msm/msm_gem.c | 23 +++++++++++++++++++++++
>>> drivers/gpu/drm/msm/msm_gem_prime.c | 15 +++++++++++++++
>>> include/uapi/drm/msm_drm.h | 14 ++++++++++++++
>>> 4 files changed, 53 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
>>> index b77fd2c531c3..b0add236cbb3 100644
>>> --- a/drivers/gpu/drm/msm/msm_drv.h
>>> +++ b/drivers/gpu/drm/msm/msm_drv.h
>>> @@ -246,6 +246,7 @@ int msm_gem_prime_vmap(struct drm_gem_object *obj, struct iosys_map *map);
>>> void msm_gem_prime_vunmap(struct drm_gem_object *obj, struct iosys_map *map);
>>> struct drm_gem_object *msm_gem_prime_import_sg_table(struct drm_device *dev,
>>> struct dma_buf_attachment *attach, struct sg_table *sg);
>>> +struct dma_buf *msm_gem_prime_export(struct drm_gem_object *obj, int flags);
>>> int msm_gem_prime_pin(struct drm_gem_object *obj);
>>> void msm_gem_prime_unpin(struct drm_gem_object *obj);
>>>
>>> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
>>> index 3708d4579203..d0f44c981351 100644
>>> --- a/drivers/gpu/drm/msm/msm_gem.c
>>> +++ b/drivers/gpu/drm/msm/msm_gem.c
>>> @@ -532,6 +532,9 @@ static int get_and_pin_iova_range_locked(struct drm_gem_object *obj,
>>>
>>> msm_gem_assert_locked(obj);
>>>
>>> + if (to_msm_bo(obj)->flags & MSM_BO_NO_SHARE)
>>> + return -EINVAL;
>>> +
>>> vma = get_vma_locked(obj, vm, range_start, range_end);
>>> if (IS_ERR(vma))
>>> return PTR_ERR(vma);
>>> @@ -1060,6 +1063,16 @@ static void msm_gem_free_object(struct drm_gem_object *obj)
>>> put_pages(obj);
>>> }
>>>
>>> + if (msm_obj->flags & MSM_BO_NO_SHARE) {
>>> + struct drm_gem_object *r_obj =
>>> + container_of(obj->resv, struct drm_gem_object, _resv);
>>> +
>>> + BUG_ON(obj->resv == &obj->_resv);
>>> +
>>> + /* Drop reference we hold to shared resv obj: */
>>> + drm_gem_object_put(r_obj);
>>> + }
>>> +
>>> drm_gem_object_release(obj);
>>>
>>> kfree(msm_obj->metadata);
>>> @@ -1092,6 +1105,15 @@ int msm_gem_new_handle(struct drm_device *dev, struct drm_file *file,
>>> if (name)
>>> msm_gem_object_set_name(obj, "%s", name);
>>>
>>> + if (flags & MSM_BO_NO_SHARE) {
>>> + struct msm_context *ctx = file->driver_priv;
>>> + struct drm_gem_object *r_obj = drm_gpuvm_resv_obj(ctx->vm);
>>> +
>>> + drm_gem_object_get(r_obj);
>>> +
>>> + obj->resv = r_obj->resv;
>>> + }
>>> +
>>> ret = drm_gem_handle_create(file, obj, handle);
>>>
>>> /* drop reference from allocate - handle holds it now */
>>> @@ -1124,6 +1146,7 @@ static const struct drm_gem_object_funcs msm_gem_object_funcs = {
>>> .free = msm_gem_free_object,
>>> .open = msm_gem_open,
>>> .close = msm_gem_close,
>>> + .export = msm_gem_prime_export,
>>> .pin = msm_gem_prime_pin,
>>> .unpin = msm_gem_prime_unpin,
>>> .get_sg_table = msm_gem_prime_get_sg_table,
>>> diff --git a/drivers/gpu/drm/msm/msm_gem_prime.c b/drivers/gpu/drm/msm/msm_gem_prime.c
>>> index ee267490c935..1a6d8099196a 100644
>>> --- a/drivers/gpu/drm/msm/msm_gem_prime.c
>>> +++ b/drivers/gpu/drm/msm/msm_gem_prime.c
>>> @@ -16,6 +16,9 @@ struct sg_table *msm_gem_prime_get_sg_table(struct drm_gem_object *obj)
>>> struct msm_gem_object *msm_obj = to_msm_bo(obj);
>>> int npages = obj->size >> PAGE_SHIFT;
>>>
>>> + if (msm_obj->flags & MSM_BO_NO_SHARE)
>>> + return ERR_PTR(-EINVAL);
>>> +
>>> if (WARN_ON(!msm_obj->pages)) /* should have already pinned! */
>>> return ERR_PTR(-ENOMEM);
>>>
>>> @@ -45,6 +48,15 @@ struct drm_gem_object *msm_gem_prime_import_sg_table(struct drm_device *dev,
>>> return msm_gem_import(dev, attach->dmabuf, sg);
>>> }
>>>
>>> +
>>> +struct dma_buf *msm_gem_prime_export(struct drm_gem_object *obj, int flags)
>>> +{
>>> + if (to_msm_bo(obj)->flags & MSM_BO_NO_SHARE)
>>> + return ERR_PTR(-EPERM);
>>> +
>>> + return drm_gem_prime_export(obj, flags);
>>> +}
>>> +
>>> int msm_gem_prime_pin(struct drm_gem_object *obj)
>>> {
>>> struct page **pages;
>>> @@ -53,6 +65,9 @@ int msm_gem_prime_pin(struct drm_gem_object *obj)
>>> if (obj->import_attach)
>>> return 0;
>>>
>>> + if (to_msm_bo(obj)->flags & MSM_BO_NO_SHARE)
>>> + return -EINVAL;
>>> +
>>> pages = msm_gem_pin_pages_locked(obj);
>>> if (IS_ERR(pages))
>>> ret = PTR_ERR(pages);
>>> diff --git a/include/uapi/drm/msm_drm.h b/include/uapi/drm/msm_drm.h
>>> index b974f5a24dbc..1bccc347945c 100644
>>> --- a/include/uapi/drm/msm_drm.h
>>> +++ b/include/uapi/drm/msm_drm.h
>>> @@ -140,6 +140,19 @@ struct drm_msm_param {
>>>
>>> #define MSM_BO_SCANOUT 0x00000001 /* scanout capable */
>>> #define MSM_BO_GPU_READONLY 0x00000002
>>> +/* Private buffers do not need to be explicitly listed in the SUBMIT
>>> + * ioctl, unless referenced by a drm_msm_gem_submit_cmd. Private
>>> + * buffers may NOT be imported/exported or used for scanout (or any
>>> + * other situation where buffers can be indefinitely pinned, but
>>> + * cases other than scanout are all kernel owned BOs which are not
>>> + * visible to userspace).
>>
>> Why is pinning for scanout a problem with those?
>>
>> Maybe I missed something but for other drivers that doesn't seem to be a problem.
>
> I guess _technically_ it could be ok because we track pin-count
> separately from dma_resv. But the motivation for that statement was
> simply that _NO_SHARE buffers share a resv obj with the VM, so they
> should not be used in a different VM (in this case, the display, which
> has it's own VM).
Ah, yes that makes perfect sense.
You should indeed avoid importing the BO into a different VM when it shares the reservation object with it. That will only cause trouble.
But at least amdgpu/radeon and I think i915 as well don't need to do that. Scanout is just separate from all VMs.
Regards,
Christian.
>
> BR,
> -R
>
>> Regards,
>> Christian.
>>
>>
>>> + *
>>> + * In exchange for those constraints, all private BOs associated with
>>> + * a single context (drm_file) share a single dma_resv, and if there
>>> + * has been no eviction since the last submit, there are no per-BO
>>> + * bookeeping to do, significantly cutting the SUBMIT overhead.
>>> + */
>>> +#define MSM_BO_NO_SHARE 0x00000004
>>> #define MSM_BO_CACHE_MASK 0x000f0000
>>> /* cache modes */
>>> #define MSM_BO_CACHED 0x00010000
>>> @@ -149,6 +162,7 @@ struct drm_msm_param {
>>>
>>> #define MSM_BO_FLAGS (MSM_BO_SCANOUT | \
>>> MSM_BO_GPU_READONLY | \
>>> + MSM_BO_NO_SHARE | \
>>> MSM_BO_CACHE_MASK)
>>>
>>> struct drm_msm_gem_new {
>>
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