[Intel-gfx] [PATCH 4/5] drm/i915: Make scatter-gather helper available to the driver

[Daniel Vetter]

On Thu, Oct 30, 2014 at 04:39:37PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> It will be used by other call sites shortly.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_dma.c         | 38 +++++++++++++++++++++++++++++
>  drivers/gpu/drm/i915/i915_gem_userptr.c | 43 ++-------------------------------
>  drivers/gpu/drm/i915/intel_drv.h        |  4 +++
>  3 files changed, 44 insertions(+), 41 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
> index 5b157bb..0b34571 100644
> --- a/drivers/gpu/drm/i915/i915_dma.c
> +++ b/drivers/gpu/drm/i915/i915_dma.c
> @@ -2070,3 +2070,41 @@ int i915_driver_device_is_agp(struct drm_device *dev)
>  {
>  	return 1;
>  }
> +
> +#if IS_ENABLED(CONFIG_SWIOTLB)
> +#define swiotlb_active() swiotlb_nr_tbl()
> +#else
> +#define swiotlb_active() 0
> +#endif
> +
> +int i915_st_set_pages(struct sg_table **st, struct page **pvec, int num_pages)
> +{
> +	struct scatterlist *sg;
> +	int ret, n;
> +
> +	*st = kmalloc(sizeof(**st), GFP_KERNEL);
> +	if (*st == NULL)
> +		return -ENOMEM;
> +
> +	if (swiotlb_active()) {
> +		ret = sg_alloc_table(*st, num_pages, GFP_KERNEL);
> +		if (ret)
> +			goto err;
> +
> +		for_each_sg((*st)->sgl, sg, num_pages, n)
> +			sg_set_page(sg, pvec[n], PAGE_SIZE, 0);
> +	} else {
> +		ret = sg_alloc_table_from_pages(*st, pvec, num_pages,
> +						0, num_pages << PAGE_SHIFT,
> +						GFP_KERNEL);
> +		if (ret)
> +			goto err;
> +	}

Ok, I don't really understand why we don't just always use
sg_alloc_table_from_pages undconditionally - if swiotlb _ever_ gets in
between us and the hw, everything will pretty much fall apart.

git blame doesn't shed light on this particular issue. Chris?
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch