[Intel-gfx] [RFC 3/5] drm/i915: Add support for CPU mapping to DRM_IOCTL_I915_GEM_MMAP_GTT

[Chris Wilson]

On Tue, Jan 26, 2016 at 02:53:31PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 96 ++++++++++++++++++++++++++++++++++++++---
>  include/uapi/drm/i915_drm.h     |  3 ++
>  2 files changed, 93 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index dacf6a0013c5..039d55a49fc6 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1954,6 +1954,60 @@ out:
>  	return i915_gem_ret_to_vm_ret(dev_priv, ret);
>  }
>  
> +static int
> +i915_gem_cpu_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> +{
> +	struct drm_i915_gem_object *obj = to_intel_bo(vma->vm_private_data);
> +	struct drm_device *dev = obj->base.dev;
> +	struct drm_i915_private *dev_priv = dev->dev_private;
> +	bool write = !!(vmf->flags & FAULT_FLAG_WRITE);
> +	pgoff_t page_offset;
> +	struct page *page;
> +	int ret;
> +
> +	/* We don't use vmf->pgoff since that has the fake offset */
> +	page_offset = ((unsigned long)vmf->virtual_address - vma->vm_start) >>
> +			PAGE_SHIFT;
> +
> +	trace_i915_gem_object_fault(obj, page_offset, true, write);
> +
> +	intel_runtime_pm_get(dev_priv);
> +
> +	ret = i915_mutex_lock_interruptible(dev);
> +	if (ret)
> +		goto out;
> +
> +	ret = i915_gem_object_set_to_cpu_domain(obj, write);
> +	if (ret)
> +		goto out_unlock;

That was a mistake in the GTT gem_fault(). If you do this, we also want
the nonblocking wait for obvious reasons.

> +	ret = i915_gem_object_get_pages(obj);
> +	if (ret)
> +		goto out_unlock;
> +
> +	page = i915_gem_object_get_page(obj, page_offset);
> +	if (!page) {
> +		ret = -ERANGE;
> +		goto out_unlock;
> +	}
> +
> +	mutex_unlock(&dev->struct_mutex);
> +
> +	ret = vm_insert_pfn(vma, (unsigned long)vmf->virtual_address,
> +			    page_to_pfn(page));

We don't have a page ref at this point, so this obj+page could be
freed (via the shrinker at least) before we insert it.

I would also be more interested in having a version that faulted the
entire object at once - though maybe we will see more random access in
future.

> +	intel_runtime_pm_put(dev_priv);
> +
> +	return i915_gem_ret_to_vm_ret(dev_priv, ret);
> +
> +out_unlock:
> +	mutex_unlock(&dev->struct_mutex);
> +out:
> +	intel_runtime_pm_put(dev_priv);
> +
> +	return i915_gem_ret_to_vm_ret(dev_priv, ret);
> +}
> +
>  /**
>   * i915_gem_release_mmap - remove physical page mappings
>   * @obj: obj in question
> @@ -2078,11 +2132,18 @@ static void i915_gem_object_free_mmap_offset(struct drm_i915_gem_object *obj)
>  	drm_gem_free_mmap_offset(&obj->base);
>  }
>  
> -int
> -i915_gem_mmap_gtt(struct drm_file *file,
> -		  struct drm_device *dev,
> -		  uint32_t handle,
> -		  uint64_t *offset)
> +static const struct vm_operations_struct i915_gem_cpu_vm_ops = {
> +	.fault = i915_gem_cpu_fault,
> +	.open = drm_gem_vm_open,
> +	.close = drm_gem_vm_close,
> +};
> +
> +static int
> +i915_gem_mmap(struct drm_file *file,
> +	      struct drm_device *dev,
> +	      uint32_t handle,
> +	      uint32_t flags,
> +	      uint64_t *offset)
>  {
>  	struct drm_i915_gem_object *obj;
>  	int ret;
> @@ -2103,10 +2164,23 @@ i915_gem_mmap_gtt(struct drm_file *file,
>  		goto out;
>  	}
>  
> +	if (!obj->base.filp && (flags & I915_MMAP2_CPU)) {
> +		DRM_DEBUG("Attempting to mmap non-shm based object via CPU!\n");
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
>  	ret = i915_gem_object_create_mmap_offset(obj);
>  	if (ret)
>  		goto out;
>  
> +	if (flags & I915_MMAP2_CPU) {
> +		ret = drm_vma_node_set_vm_ops(&obj->base.vma_node,
> +					      &i915_gem_cpu_vm_ops);
> +		if (ret)
> +			goto out;
> +	}

We would also need a WC equivalent.

It looks fairly sane. I wanted this just a short while ago, but figured
out a way of using regular mmap() to give me the inheritance instead.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre