Re: [PATCH] drm/vgem: fix cache synchronization on arm/arm64

[Sean Paul <sean@poorly.run>]

On Fri, Jul 19, 2019 at 11:21:53AM +0200, Daniel Vetter wrote:
> On Wed, Jul 17, 2019 at 02:15:37PM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@chromium.org>
> > 
> > drm_cflush_pages() is no-op on arm/arm64.  But instead we can use
> > dma_sync API.
> > 
> > Fixes failures w/ vgem_test.
> > 
> > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > ---
> > An alternative approach to the series[1] I sent yesterday
> > 
> > On the plus side, it keeps the WC buffers and avoids any drm core
> > changes.  On the minus side, I don't think it will work (at least
> > on arm64) prior to v5.0[2], so the fix can't be backported very
> > far.
> 
> Yeah seems a lot more reasonable.
> 
> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Applied to drm-misc-fixes, thanks!

Sean

> > 
> > [1] https://patchwork.freedesktop.org/series/63771/
> > [2] depends on 356da6d0cde3323236977fce54c1f9612a742036
> > 
> >  drivers/gpu/drm/vgem/vgem_drv.c | 130 ++++++++++++++++++++------------
> >  1 file changed, 83 insertions(+), 47 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
> > index 76d95b5e289c..6c9b5e20b3d4 100644
> > --- a/drivers/gpu/drm/vgem/vgem_drv.c
> > +++ b/drivers/gpu/drm/vgem/vgem_drv.c
> > @@ -47,10 +47,16 @@ static struct vgem_device {
> >  	struct platform_device *platform;
> >  } *vgem_device;
> >  
> > +static void sync_and_unpin(struct drm_vgem_gem_object *bo);
> > +static struct page **pin_and_sync(struct drm_vgem_gem_object *bo);
> > +
> >  static void vgem_gem_free_object(struct drm_gem_object *obj)
> >  {
> >  	struct drm_vgem_gem_object *vgem_obj = to_vgem_bo(obj);
> >  
> > +	if (!obj->import_attach)
> > +		sync_and_unpin(vgem_obj);
> > +
> >  	kvfree(vgem_obj->pages);
> >  	mutex_destroy(&vgem_obj->pages_lock);
> >  
> > @@ -78,40 +84,15 @@ static vm_fault_t vgem_gem_fault(struct vm_fault *vmf)
> >  		return VM_FAULT_SIGBUS;
> >  
> >  	mutex_lock(&obj->pages_lock);
> > +	if (!obj->pages)
> > +		pin_and_sync(obj);
> >  	if (obj->pages) {
> >  		get_page(obj->pages[page_offset]);
> >  		vmf->page = obj->pages[page_offset];
> >  		ret = 0;
> >  	}
> >  	mutex_unlock(&obj->pages_lock);
> > -	if (ret) {
> > -		struct page *page;
> > -
> > -		page = shmem_read_mapping_page(
> > -					file_inode(obj->base.filp)->i_mapping,
> > -					page_offset);
> > -		if (!IS_ERR(page)) {
> > -			vmf->page = page;
> > -			ret = 0;
> > -		} else switch (PTR_ERR(page)) {
> > -			case -ENOSPC:
> > -			case -ENOMEM:
> > -				ret = VM_FAULT_OOM;
> > -				break;
> > -			case -EBUSY:
> > -				ret = VM_FAULT_RETRY;
> > -				break;
> > -			case -EFAULT:
> > -			case -EINVAL:
> > -				ret = VM_FAULT_SIGBUS;
> > -				break;
> > -			default:
> > -				WARN_ON(PTR_ERR(page));
> > -				ret = VM_FAULT_SIGBUS;
> > -				break;
> > -		}
> >  
> > -	}
> >  	return ret;
> >  }
> >  
> > @@ -277,32 +258,93 @@ static const struct file_operations vgem_driver_fops = {
> >  	.release	= drm_release,
> >  };
> >  
> > -static struct page **vgem_pin_pages(struct drm_vgem_gem_object *bo)
> > +/* Called under pages_lock, except in free path (where it can't race): */
> > +static void sync_and_unpin(struct drm_vgem_gem_object *bo)
> >  {
> > -	mutex_lock(&bo->pages_lock);
> > -	if (bo->pages_pin_count++ == 0) {
> > -		struct page **pages;
> > +	struct drm_device *dev = bo->base.dev;
> > +
> > +	if (bo->table) {
> > +		dma_sync_sg_for_cpu(dev->dev, bo->table->sgl,
> > +				bo->table->nents, DMA_BIDIRECTIONAL);
> > +		sg_free_table(bo->table);
> > +		kfree(bo->table);
> > +		bo->table = NULL;
> > +	}
> > +
> > +	if (bo->pages) {
> > +		drm_gem_put_pages(&bo->base, bo->pages, true, true);
> > +		bo->pages = NULL;
> > +	}
> > +}
> > +
> > +static struct page **pin_and_sync(struct drm_vgem_gem_object *bo)
> > +{
> > +	struct drm_device *dev = bo->base.dev;
> > +	int npages = bo->base.size >> PAGE_SHIFT;
> > +	struct page **pages;
> > +	struct sg_table *sgt;
> > +
> > +	WARN_ON(!mutex_is_locked(&bo->pages_lock));
> > +
> > +	pages = drm_gem_get_pages(&bo->base);
> > +	if (IS_ERR(pages)) {
> > +		bo->pages_pin_count--;
> > +		mutex_unlock(&bo->pages_lock);
> > +		return pages;
> > +	}
> >  
> > -		pages = drm_gem_get_pages(&bo->base);
> > -		if (IS_ERR(pages)) {
> > -			bo->pages_pin_count--;
> > -			mutex_unlock(&bo->pages_lock);
> > -			return pages;
> > -		}
> > +	sgt = drm_prime_pages_to_sg(pages, npages);
> > +	if (IS_ERR(sgt)) {
> > +		dev_err(dev->dev,
> > +			"failed to allocate sgt: %ld\n",
> > +			PTR_ERR(bo->table));
> > +		drm_gem_put_pages(&bo->base, pages, false, false);
> > +		mutex_unlock(&bo->pages_lock);
> > +		return ERR_CAST(bo->table);
> > +	}
> > +
> > +	/*
> > +	 * Flush the object from the CPU cache so that importers
> > +	 * can rely on coherent indirect access via the exported
> > +	 * dma-address.
> > +	 */
> > +	dma_sync_sg_for_device(dev->dev, sgt->sgl,
> > +			sgt->nents, DMA_BIDIRECTIONAL);
> > +
> > +	bo->pages = pages;
> > +	bo->table = sgt;
> > +
> > +	return pages;
> > +}
> > +
> > +static struct page **vgem_pin_pages(struct drm_vgem_gem_object *bo)
> > +{
> > +	struct page **pages;
> >  
> > -		bo->pages = pages;
> > +	mutex_lock(&bo->pages_lock);
> > +	if (bo->pages_pin_count++ == 0 && !bo->pages) {
> > +		pages = pin_and_sync(bo);
> > +	} else {
> > +		WARN_ON(!bo->pages);
> > +		pages = bo->pages;
> >  	}
> >  	mutex_unlock(&bo->pages_lock);
> >  
> > -	return bo->pages;
> > +	return pages;
> >  }
> >  
> >  static void vgem_unpin_pages(struct drm_vgem_gem_object *bo)
> >  {
> > +	/*
> > +	 * We shouldn't hit this for imported bo's.. in the import
> > +	 * case we don't own the scatter-table
> > +	 */
> > +	WARN_ON(bo->base.import_attach);
> > +
> >  	mutex_lock(&bo->pages_lock);
> >  	if (--bo->pages_pin_count == 0) {
> > -		drm_gem_put_pages(&bo->base, bo->pages, true, true);
> > -		bo->pages = NULL;
> > +		WARN_ON(!bo->table);
> > +		sync_and_unpin(bo);
> >  	}
> >  	mutex_unlock(&bo->pages_lock);
> >  }
> > @@ -310,18 +352,12 @@ static void vgem_unpin_pages(struct drm_vgem_gem_object *bo)
> >  static int vgem_prime_pin(struct drm_gem_object *obj)
> >  {
> >  	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
> > -	long n_pages = obj->size >> PAGE_SHIFT;
> >  	struct page **pages;
> >  
> >  	pages = vgem_pin_pages(bo);
> >  	if (IS_ERR(pages))
> >  		return PTR_ERR(pages);
> >  
> > -	/* Flush the object from the CPU cache so that importers can rely
> > -	 * on coherent indirect access via the exported dma-address.
> > -	 */
> > -	drm_clflush_pages(pages, n_pages);
> > -
> >  	return 0;
> >  }
> >  
> > -- 
> > 2.21.0
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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-- 
Sean Paul, Software Engineer, Google / Chromium OS