linux-kernel - Re: [PATCH 4/9] dma-mapping: move the arm64 ncoherent alloc/free support to common code

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Message-ID: <20181204142216.GB2618@lst.de>
Date:   Tue, 4 Dec 2018 15:22:16 +0100
From:   Christoph Hellwig <hch@....de>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Guo Ren <ren_guo@...ky.com>,
        Laura Abbott <labbott@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 4/9] dma-mapping: move the arm64 ncoherent alloc/free
 support to common code

> > +int __init dma_atomic_pool_init(gfp_t gfp, pgprot_t prot)
> > +{
> > +	unsigned int pool_size_order = get_order(atomic_pool_size);
> > +	unsigned long nr_pages = atomic_pool_size >> PAGE_SHIFT;
> > +	struct page *page;
> > +	void *addr;
> > +	int ret;
> > +
> > +	if (dev_get_cma_area(NULL))
> > +		page = dma_alloc_from_contiguous(NULL, nr_pages,
> > +						 pool_size_order, false);
> > +	else
> > +		page = alloc_pages(gfp, pool_size_order);
> > +	if (!page)
> > +		goto out;
> > +
> > +	memset(page_address(page), 0, atomic_pool_size);
> 
> Note that this won't work if 'page' is a highmem page - should there
> be a check for that, or a check for the gfp flags?
>
> Also, is this memset() actually useful, or a waste of cycles - when we
> allocate from this pool (see dma_alloc_from_pool()), we always memset()
> the buffer.

Currently there is no user that supports highmem, but yes, the memset
should probably simply be removed.