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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 26/01/16 15:10, Chris Wilson wrote:
> On Tue, Jan 26, 2016 at 02:53:31PM +0000, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@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.

You suggest leaving it for userspace?

And how would a non-blocking wait work?

>
>> +	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.

Oh yeah, need to pin the pages..

> 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.

Yeah I did not want to concern myself with more code since this was a 
proof of concept anyway.

>> +	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.

So would it be useful to cleanup and finish this work or not?

Regards,

Tvrtko
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