Re: [PATCH 17/18] dma-iommu: implement ->alloc_noncoherent

[Christoph Hellwig]

On Fri, Sep 25, 2020 at 06:46:22PM +0000, Tomasz Figa wrote:
> > +static void *iommu_dma_alloc_noncoherent(struct device *dev, size_t size,
> > +		dma_addr_t *handle, enum dma_data_direction dir, gfp_t gfp)
> > +{
> > +	if (!gfpflags_allow_blocking(gfp)) {
> > +		struct page *page;
> > +
> > +		page = dma_common_alloc_pages(dev, size, handle, dir, gfp);
> > +		if (!page)
> > +			return NULL;
> > +		return page_address(page);
> > +	}
> > +
> > +	return iommu_dma_alloc_remap(dev, size, handle, gfp | __GFP_ZERO,
> > +				     PAGE_KERNEL, 0);
> 
> iommu_dma_alloc_remap() makes use of the DMA_ATTR_ALLOC_SINGLE_PAGES attribute
> to optimize the allocations for devices which don't care about how contiguous
> the backing memory is. Do you think we could add an attrs argument to this
> function and pass it there?
> 
> As ARM is being moved to the common iommu-dma layer as well, we'll probably
> make use of the argument to support the DMA_ATTR_NO_KERNEL_MAPPING attribute to
> conserve the vmalloc area.

We could probably at it.  However I wonder why this is something the
drivers should care about.  Isn't this really something that should
be a kernel-wide policy for a given system?