linux-kernel - Re: [RFC PATCH] drivers: dma-mapping: Do not attempt to create a scatterlist for from

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Message-ID: <20170425180525.GT17774@n2100.armlinux.org.uk>
Date:   Tue, 25 Apr 2017 19:05:26 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        geert@...ux-m68k.org, a.hajda@...sung.com, robin.murphy@....com,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCH] drivers: dma-mapping: Do not attempt to create a
 scatterlist for from_coherent buffers

On Tue, Apr 25, 2017 at 06:57:39PM +0100, Catalin Marinas wrote:
> Memory returned by dma_alloc_from_coherent() is not backed by struct
> page and creating a scatterlist would use invalid page pointers. The
> patch introduces the dma_vaddr_from_coherent() function and the
> corresponding check in dma_get_sgtable_attrs().
> 
> Fixes: d2b7428eb0ca ("common: dma-mapping: introduce dma_get_sgtable() function")
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Russell King - ARM Linux <linux@....linux.org.uk>
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> ---
> 
> In a recent discussion around the iommu DMA ops on arm64, Russell
> pointed out that dma_get_sgtable is not safe since the coherent DMA
> memory is not always backed by struct page. Russell has queued an
> arm-specific patch checking for pfn_valid() but I thought I'd make a
> more generic fix. This patch aims to bring the dma_get_sgtable() API in
> line with the dma_alloc/mmap/free with respect to the from_coherent
> memory.

Sorry, I don't think this is the correct approach.

You're assuming that 'vaddr' will always be a valid lowmem address.
That isn't always the case - some dma coherent allocations provide
remapped memory.

The reason for dma_get_sgtable() existing is to coerce the DMA
coherent memory into a scatterlist so that it can be passed through
the dma_buf API - that's where the problem lies.  The dma_buf API
needs fixing so that coherent memory can be sanely passed, and
dma_get_sgtable() needs to be put out of its misery.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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