From: robin.murphy@arm•com (Robin Murphy)
To: linux-arm-kernel@lists•infradead.org
Subject: [RFC PATCH v2 2/3] arm64: add IOMMU dma_ops
Date: Mon, 09 Mar 2015 20:09:49 +0000 [thread overview]
Message-ID: <54FDFE0D.8030807@arm.com> (raw)
In-Reply-To: <20150309175904.GC8656@n2100.arm.linux.org.uk>
Hi Russell,
On 09/03/15 17:59, Russell King - ARM Linux wrote:
[...]
>>>> For a noncoherent device, dma_map_single() will end up calling
>>> __dma_map_area() with the page offset and size of the original request, so
>>> the updated part gets flushed by VA, and the rest of the page isn't touched
>>> if it doesn't need to be. On the other hand if the page tables were
>>> allocated with dma_alloc_coherent() in the first place, then just calling
>>> dma_sync_single_for_device() for the updated region should suffice.
>
> That's wrong. dma_sync_single_*() is not permitted to be called on
> coherently allocated memory. Where coherent memory needs to be remapped,
> dma_sync_single_*() will panic the kernel.
>
> If it's in coherent memory, all you should need is the appropriate
> memory barrier to ensure that the DMA agent can see the writes.
You're quite right, that's the whole point of *coherent* allocations
after all. I got my syncs and barriers muddled there.
>>> Where exactly would you call the dma_unmap? It seems a bit strange to
>>> be repeatedly calling dma_map and never calling dma_unmap. I don't see it
>>> explicitly forbidden in the docs anywhere to do this but it seems like
>>> it would be violating the implicit handoff of dma_map/dma_unmap.
>>
>> I think ideally you'd call dma_map_page when you first create the page
>> table, dma_sync_single_for_device on any update, and dma_unmap_page when you
>> tear it down, and you'd also use the appropriate DMA addresses everywhere
>> instead of physical addresses.
>
> No.
>
> dma_map_page() ownership changes CPU->DMA
> dma_sync_single_for_cpu() ownership changes DMA->CPU
> dma_sync_single_for_device() ownership changes CPU->DMA
> dma_unmap_page() ownership changes DMA->CPU
>
> It's invalid to miss out the pairing that give those ownership changes.
Thanks for the clarification - the wording in DMA-API.txt rather implies
that in the DMA_TO_DEVICE case you only have to sync the updated data
/after/ writing it. For the sake of purely getting pages flushed, would
it be more reasonable then to call dma_map_single() followed immediately
by dma_unmap_single_attrs() with DMA_ATTR_SKIP_CPU_SYNC? Since we know
the IOMMU can never write back to memory (ones that can are a different
issue) it would be nice to be able to skip the extra invalidations
somehow, without too heinously abusing the API.
Robin.
next prev parent reply other threads:[~2015-03-09 20:09 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-06 14:55 [RFC PATCH v2 0/3] arm64: IOMMU-backed DMA mapping Robin Murphy
2015-02-06 14:55 ` [RFC PATCH v2 1/3] iommu: implement common IOMMU ops for " Robin Murphy
2015-02-09 4:05 ` Will Deacon
2015-02-10 15:11 ` Robin Murphy
2015-03-12 12:45 ` Marek Szyprowski
2015-02-06 14:55 ` [RFC PATCH v2 2/3] arm64: add IOMMU dma_ops Robin Murphy
2015-02-09 6:02 ` Will Deacon
2015-02-10 15:40 ` Robin Murphy
2015-02-10 4:39 ` Yingjoe Chen
2015-02-10 12:07 ` Robin Murphy
2015-02-14 8:03 ` Yong Wu
2015-02-16 20:04 ` Robin Murphy
2015-03-03 3:38 ` Yong Wu
2015-03-03 12:15 ` Robin Murphy
2015-03-05 0:19 ` Laura Abbott
2015-03-05 11:16 ` Robin Murphy
2015-03-09 17:59 ` Russell King - ARM Linux
2015-03-09 20:09 ` Robin Murphy [this message]
2015-03-10 10:16 ` Robin Murphy
2015-03-12 12:50 ` Marek Szyprowski
2015-02-06 14:55 ` [RFC PATCH v2 3/3] arm64: hook up " Robin Murphy
2015-03-03 11:05 ` leizhen
2015-03-03 13:10 ` Robin Murphy
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=54FDFE0D.8030807@arm.com \
--to=robin.murphy@arm$(echo .)com \
--cc=linux-arm-kernel@lists$(echo .)infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox