Re: [PATCH 5/6] drm/i915: Support for pread/pwrite from/to non shmem backed objects

[Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>]

On Thu, 2015-12-10 at 16:24 +0530, Ankitprasad Sharma wrote:
Missed Chris in last mail, adding him
On Wed, 2015-12-09 at 16:15 +0000, Tvrtko Ursulin wrote:
> Hi,
> 
> On 09/12/15 12:46, ankitprasad.r.sharma@intel.com wrote:
> > From: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> >
> > This patch adds support for extending the pread/pwrite functionality
> > for objects not backed by shmem. The access will be made through
> > gtt interface. This will cover objects backed by stolen memory as well
> > as other non-shmem backed objects.
> >
> > v2: Drop locks around slow_user_access, prefault the pages before
> > access (Chris)
> >
> > v3: Rebased to the latest drm-intel-nightly (Ankit)
> >
> > v4: Moved page base & offset calculations outside the copy loop,
> > corrected data types for size and offset variables, corrected if-else
> > braces format (Tvrtko/kerneldocs)
> >
> > v5: Enabled pread/pwrite for all non-shmem backed objects including
> > without tiling restrictions (Ankit)
> >
> > v6: Using pwrite_fast for non-shmem backed objects as well (Chris)
> >
> > v7: Updated commit message, Renamed i915_gem_gtt_read to i915_gem_gtt_copy,
> > added pwrite slow path for non-shmem backed objects (Chris/Tvrtko)
> >
> > v8: Updated v7 commit message, mutex unlock around pwrite slow path for
> > non-shmem backed objects (Tvrtko)
> >
> > Testcase: igt/gem_stolen
> >
> > Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_gem.c | 151 +++++++++++++++++++++++++++++++++-------
> >   1 file changed, 127 insertions(+), 24 deletions(-)> >

> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index ed97de6..68ed67a 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -614,6 +614,99 @@ shmem_pread_slow(struct page *page, int shmem_page_offset, int page_length,
> >   	return ret ? - EFAULT : 0;
> >   }
> >
> > +static inline uint64_t
> > +slow_user_access(struct io_mapping *mapping,
> > +		 uint64_t page_base, int page_offset,
> > +		 char __user *user_data,
> > +		 int length, bool pwrite)
> > +{
> > +	void __iomem *vaddr_inatomic;
> > +	void *vaddr;
> > +	uint64_t unwritten;
> > +
> > +	vaddr_inatomic = io_mapping_map_wc(mapping, page_base);
> > +	/* We can use the cpu mem copy function because this is X86. */
> > +	vaddr = (void __force *)vaddr_inatomic + page_offset;
> > +	if (pwrite)
> > +		unwritten = __copy_from_user(vaddr, user_data, length);
> > +	else
> > +		unwritten = __copy_to_user(user_data, vaddr, length);
> > +
> > +	io_mapping_unmap(vaddr_inatomic);
> > +	return unwritten;
> > +}
> > +
> > +static int
> > +i915_gem_gtt_copy(struct drm_device *dev,
> > +		   struct drm_i915_gem_object *obj, uint64_t size,
> > +		   uint64_t data_offset, uint64_t data_ptr)
> > +{
> > +	struct drm_i915_private *dev_priv = dev->dev_private;
> > +	char __user *user_data;
> > +	uint64_t remain;
> > +	uint64_t offset, page_base;
> > +	int page_offset, page_length, ret = 0;
> > +
> > +	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE);
> > +	if (ret)
> > +		goto out;
> > +
> > +	ret = i915_gem_object_set_to_gtt_domain(obj, false);
> > +	if (ret)
> > +		goto out_unpin;
> > +
> > +	ret = i915_gem_object_put_fence(obj);
> > +	if (ret)
> > +		goto out_unpin;
> > +
> > +	user_data = to_user_ptr(data_ptr);
> > +	remain = size;
> > +	offset = i915_gem_obj_ggtt_offset(obj) + data_offset;
> > +
> > +	mutex_unlock(&dev->struct_mutex);
> > +	if (likely(!i915.prefault_disable))
> > +		ret = fault_in_multipages_writeable(user_data, remain);
> > +
> > +	/*
> > +	 * page_offset = offset within page
> > +	 * page_base = page offset within aperture
> > +	 */
> > +	page_offset = offset_in_page(offset);
> > +	page_base = offset & PAGE_MASK;
> > +
> > +	while (remain > 0) {
> > +		/* page_length = bytes to copy for this page */
> > +		page_length = remain;
> > +		if ((page_offset + remain) > PAGE_SIZE)
> > +			page_length = PAGE_SIZE - page_offset;
> > +
> > +		/* This is a slow read/write as it tries to read from
> > +		 * and write to user memory which may result into page
> > +		 * faults
> > +		 */
> > +		ret = slow_user_access(dev_priv->gtt.mappable, page_base,
> > +				       page_offset, user_data,
> > +				       page_length, false);
> > +
> > +		if (ret) {
> > +			ret = -EFAULT;
> > +			break;
> > +		}
> > +
> > +		remain -= page_length;
> > +		user_data += page_length;
> > +		page_base += page_length;
> > +		page_offset = 0;
> > +	}
> > +
> > +	mutex_lock(&dev->struct_mutex);
> > +
> > +out_unpin:
> > +	i915_gem_object_ggtt_unpin(obj);
> > +out:
> > +	return ret;
> > +}
> > +
> >   static int
> >   i915_gem_shmem_pread(struct drm_device *dev,
> >   		     struct drm_i915_gem_object *obj,
> > @@ -737,17 +830,14 @@ i915_gem_pread_ioctl(struct drm_device *dev, void *data,
> >   		goto out;
> >   	}
> >
> > -	/* prime objects have no backing filp to GEM pread/pwrite
> > -	 * pages from.
> > -	 */
> > -	if (!obj->base.filp) {
> > -		ret = -EINVAL;
> > -		goto out;
> > -	}
> > -
> >   	trace_i915_gem_object_pread(obj, args->offset, args->size);
> >
> > -	ret = i915_gem_shmem_pread(dev, obj, args, file);
> > +	/* pread for non shmem backed objects */
> > +	if (!obj->base.filp && obj->tiling_mode == I915_TILING_NONE)
> > +		ret = i915_gem_gtt_copy(dev, obj, args->size,
> > +					args->offset, args->data_ptr);
> > +	else
> > +		ret = i915_gem_shmem_pread(dev, obj, args, file);
> 
> Hm, it will end up calling i915_gem_shmem_pread for non-shmem backed 
> objects if tiling is set. Sounds wrong to me unless I am missing something?
> 
Thanks for pointing it out, need to add a check there.
> >
> >   out:
> >   	drm_gem_object_unreference(&obj->base);
> > @@ -789,10 +879,12 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_private *i915,
> >   			 struct drm_i915_gem_pwrite *args,
> >   			 struct drm_file *file)
> >   {
> > +	struct drm_device *dev = obj->base.dev;
> >   	struct drm_mm_node node;
> >   	uint64_t remain, offset;
> >   	char __user *user_data;
> >   	int ret;
> > +	bool faulted = false;
> >
> >   	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE | PIN_NONBLOCK);
> >   	if (ret) {
> > @@ -851,11 +943,29 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_private *i915,
> >   		/* If we get a fault while copying data, then (presumably) our
> >   		 * source page isn't available.  Return the error and we'll
> >   		 * retry in the slow path.
> > +		 * If the object is non-shmem backed, we retry again with the
> > +		 * path that handles page fault.
> >   		 */
> > -		if (fast_user_write(i915->gtt.mappable, page_base,
> > -				    page_offset, user_data, page_length)) {
> > -			ret = -EFAULT;
> > -			goto out_flush;
> > +		if (faulted || fast_user_write(i915->gtt.mappable,
> > +						page_base, page_offset,
> > +						user_data, page_length)) {
> > +			if (!obj->base.filp) {
> > +				faulted = true;
> > +				mutex_unlock(&dev->struct_mutex);
> > +				if (slow_user_access(i915->gtt.mappable,
> > +						     page_base,
> > +						     page_offset, user_data,
> > +						     page_length, true)) {
> > +					ret = -EFAULT;
> > +					mutex_lock(&dev->struct_mutex);
> > +					goto out_flush;
> > +				}
> > +
> > +				mutex_lock(&dev->struct_mutex);
> > +			} else {
> > +				ret = -EFAULT;
> > +				goto out_flush;
> > +			}
> >   		}
> 
> Some questions:
> 
> 1. What is the advantage of doing the slow access for non-shmem backed 
> objects inside a single loop, as opposed to extracting it in a separate 
> function?
> 
> For example i915_gem_gtt_pwrite_slow ? Then it could have been called 
> from i915_gem_pwrite_ioctl depending on the master if statement there, > fallback etc.
> 
> I think it would be clearer unless there is a special reason it makes 
> sense to go with the fast path first and then switch to slow path at the 
> point first fault is hit.
I am ready for any of the approach, but Chris suggested to extend
pwrite_fast as it is already being used for faster pwrites.

Chris,
Would it be better to do a pwrite to non-shmem backed objects via a
separate function?

 
> 2. I have noticed the shmem pwrite slowpath makes explicit mention of 
> potential changes to the object domain while the lock was dropped and 
> takes care of flushing the cache in that case.
> 
> Is this something this path should do as well, or if not why not?
I do not think that this path needs to take care of flushing the cache,
as for stolen backed objects are not accessible to CPU hence no
possibility of it being in the CPU cache atleast for the stolen-backed
objects. 
For other non-shmem backed objects (dmabuf, usrptr, phys), may need some
inputs from Chris on how to handle it.

Thanks, Ankit

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