linux-kernel - Re: [PATCH v2 07/19] OpenRISC: DMA

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Message-Id: <201107051737.30120.arnd@arndb.de>
Date:	Tue, 5 Jul 2011 17:37:29 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Jonas Bonn <jonas@...thpole.se>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 07/19] OpenRISC: DMA

On Saturday 02 July 2011, Jonas Bonn wrote:
> +void *or1k_dma_alloc_coherent(struct device *dev, size_t size,
> +                             dma_addr_t *dma_handle, gfp_t flag)
> +{
> +       int order;
> +       unsigned long page, va;
> +       pgprot_t prot;
> +       struct vm_struct *area;
> +
> +       /* Only allocate page size areas. */
> +       size = PAGE_ALIGN(size);
> +       order = get_order(size);
> +
> +       page = __get_free_pages(flag, order);
> +       if (!page)
> +               return NULL;
> +
> +       /* Allocate some common virtual space to map the new pages. */
> +       area = get_vm_area(size, VM_ALLOC);
> +       if (area == NULL) {
> +               free_pages(page, order);
> +               return NULL;
> +       }
> +       va = (unsigned long)area->addr;
> +
> +       /* This gives us the real physical address of the first page. */
> +       *dma_handle = __pa(page);
> +
> +       prot = PAGE_KERNEL_NOCACHE;
> +
> +       /* This isn't so much ioremap as just simply 'remap' */
> +       if (ioremap_page_range(va, va + size, *dma_handle, prot)) {
> +               vfree(area->addr);
> +               return NULL;
> +       }
> +
> +       return (void *)va;
> +}

This will result in having conflicting mappings, one with and another
without caching, which a lot of CPU architectures don't like. Are you
sure that you can handle this with or1k?

I think at the very least you will need to flush the cache for
the linear mapping, to avoid writing back dirty cache lines over
the DMA buffer.

You can save a little memory by using alloc_pages_exact instead of
get_free_pages, which always gives you a power-of-two size.

Also, isn't get_vm_area+ioremap_page_range the same as ioremap
on or1k?

In the case that ioremap_page_ranges fails, I think you have a
memory leak, or worse, because areas is not backed by the pages at that
moment.

	Arnd
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