From: Christoph Hellwig <hch@infradead.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Christoph Hellwig <hch@infradead.org>,
Vladimir Murzin <vladimir.murzin@arm.com>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, linux@armlinux.org.uk, sza@esh.hu,
arnd@arndb.de, gregkh@linuxfoundation.org,
akpm@linux-foundation.org, alexandre.torgue@st.com,
benjamin.gaignard@linaro.org, kbuild-all@01.org,
Michal Nazarewicz <mina86@mina86.com>,
Marek Szyprowski <m.szyprowski@samsung.com>,
Rob Herring <robh+dt@kernel.org>,
Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v5 4/7] drivers: dma-coherent: Introduce default DMA pool
Date: Mon, 26 Jun 2017 02:42:50 -0700 [thread overview]
Message-ID: <20170626094250.GB21570@infradead.org> (raw)
In-Reply-To: <3d5ea779-90a3-f0b5-b9aa-161d4c0855e7@arm.com>
On Tue, Jun 20, 2017 at 03:24:21PM +0100, Robin Murphy wrote:
> True, but the case here is where we need a special piece of coherent
> memory for *all* devices, and it was more complicated *not* to reuse the
> existing infrastructure. This would already be achievable by specifying
> a separate rmem carveout per device, but the shared pool just makes life
> easier, and mirrors the functionality dma-contiguous already supports.
І'm really worried about the code in dma-coherent.c - the original
version clearly intends to have a coherent pool per device, declared
in the driver. Then Marek added the reserved_mem interface, and
now we get another variant of it. Conceptually the per-device
and global pool are very different, and to me it seems like the
reserved mem should be a different interface.
> > If you're allocating out of the global allocator the memory should
> > come from the normal dma_ops ->alloc allocator - and also take
> > the attrs into account (e.g. for DMA_ATTR_NON_CONSISTENT or
> > DMA_ATTR_NO_KERNEL_MAPPING requests you don't need coherent memory)
>
> The context here is noMMU but with caches - the problem being that the
> normal allocator will give back kernel memory, and there's no way to
> make that coherent with devices short of not enabling the caches in the
> first place, which is obviously undesirable. The trick is that RAM is
> aliased (in hardware) at two addresses, one of which makes CPU accesses
> non-cacheable, so by only ever accessing the RAM set aside for the
> coherent DMA pool using the non-cacheable alias (represented by the
> dma_pfn_offset) we can achieve DMA coherency.
Yes, and I think this is something we already have to deal with
for example on mips. A simple genalloc allocator from your pool
in the normal dma_ops implementation should do the work just fine.
next prev parent reply other threads:[~2017-06-26 9:42 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-24 10:24 [PATCH v5 0/7] ARM: Fix dma_alloc_coherent() and friends for NOMMU Vladimir Murzin
2017-05-24 10:24 ` [PATCH v5 1/7] dma: Take into account dma_pfn_offset Vladimir Murzin
2017-06-20 13:24 ` Christoph Hellwig
2017-06-22 12:47 ` Vladimir Murzin
2017-05-24 10:24 ` [PATCH v5 2/7] dma: Add simple dma_noop_mmap Vladimir Murzin
2017-06-20 13:23 ` Christoph Hellwig
2017-06-22 12:46 ` Vladimir Murzin
2017-06-26 9:45 ` Christoph Hellwig
2017-05-24 10:24 ` [PATCH v5 3/7] drivers: dma-coherent: Account dma_pfn_offset when used with device tree Vladimir Murzin
2017-06-20 13:42 ` Christoph Hellwig
2017-06-20 13:50 ` Robin Murphy
2017-06-26 9:37 ` Christoph Hellwig
2017-06-22 12:59 ` Vladimir Murzin
2017-05-24 10:24 ` [PATCH v5 4/7] drivers: dma-coherent: Introduce default DMA pool Vladimir Murzin
2017-06-20 13:49 ` Christoph Hellwig
2017-06-20 14:24 ` Robin Murphy
2017-06-26 9:42 ` Christoph Hellwig [this message]
2017-06-26 14:08 ` Vladimir Murzin
2017-06-27 14:36 ` Robin Murphy
2017-06-27 15:22 ` Christoph Hellwig
2017-06-22 13:18 ` Vladimir Murzin
2017-06-26 9:44 ` Christoph Hellwig
2017-05-24 10:24 ` [PATCH v5 5/7] ARM: NOMMU: Introduce dma operations for noMMU Vladimir Murzin
2017-05-24 10:24 ` [PATCH v5 6/7] ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class cpus Vladimir Murzin
2017-05-24 10:24 ` [PATCH v5 7/7] ARM: dma-mapping: Remove traces of NOMMU code Vladimir Murzin
2017-06-08 8:28 ` [PATCH v5 0/7] ARM: Fix dma_alloc_coherent() and friends for NOMMU Vladimir Murzin
2017-06-08 16:25 ` Russell King - ARM Linux
2017-06-15 7:25 ` Vladimir Murzin
2017-06-20 12:59 ` Vladimir Murzin
2017-06-20 13:00 ` Christoph Hellwig
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