From: mike.looijmans@topic•nl (Mike Looijmans)
To: linux-arm-kernel@lists•infradead.org
Subject: dma_alloc_coherent versus streaming DMA, neither works satisfactory
Date: Fri, 08 May 2015 07:55:26 +0200 [thread overview]
Message-ID: <554C4FCE.6070802@topic.nl> (raw)
In-Reply-To: <20150507143010.GC2067@n2100.arm.linux.org.uk>
On 07-05-15 16:30, Russell King - ARM Linux wrote:
> On Thu, May 07, 2015 at 04:08:54PM +0200, Mike Looijmans wrote:
>> I read the rest of the thread, apparently it was never integrated.
>>
>> The patch for "non-consistent" is a BUG FIX, not some feature request or so.
>> I was already wondering why my driver had to kalloc pages to get proper
>> caching on it.
>
> I disagree.
>
>> From https://www.kernel.org/doc/Documentation/DMA-attributes.txt:
>> """
>> DMA_ATTR_NON_CONSISTENT ... lets the platform to choose to return either
>> consistent or non-consistent memory as it sees fit. By using this API,
>> you are guaranteeing to the platform that you have all the correct and
>> necessary sync points for this memory in the driver.
>> """
>
> DMA attributes are something that came in _after_ the DMA API had been
> around for many years. It's a "new feature" that was added to an
> existing subsystem, and because there have been no need for it to be
> implemented on ARM, the new feature was never implemented.
>
> More than that, the vast majority of ARM hardware can't provide this
> kind of memory, and there are _no_ kernel APIs to ensure that if
By "non-coherent" memory I thought it meant the same kind of memory that
kalloc would return. But from your answer it seems I am mistaken and
this is something different?
> cacheable memory were to be returned, they could issue the necessary
> cache flushes to ensure that the device could see the data.
Then what do the dma_sync_... methods do?
It has been my understanding that one can use dma_map... and dma_sync...
methods to make memory ranges visible to the device.
Using dma_sync on coherent memory is just a waste of resources. So how
do i allocate memory that I'm supposed to use with dma_sync?
> So it's _not_ a bug fix, and there have been very good reasons not to
> implement it.
--
Mike Looijmans
next prev parent reply other threads:[~2015-05-08 5:55 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-23 11:52 dma_alloc_coherent versus streaming DMA, neither works satisfactory Mike Looijmans
2015-04-23 12:32 ` Arnd Bergmann
2015-04-23 13:05 ` Mike Looijmans
2015-04-29 8:47 ` Mike Looijmans
2015-04-29 9:01 ` Arnd Bergmann
2015-04-29 9:17 ` Russell King - ARM Linux
2015-04-29 9:47 ` Mike Looijmans
2015-04-29 10:07 ` Arnd Bergmann
2015-04-29 10:33 ` Mike Looijmans
2015-04-29 10:41 ` Arnd Bergmann
2015-04-29 12:49 ` Mike Looijmans
2015-04-29 13:13 ` Arnd Bergmann
2015-04-30 13:50 ` Mike Looijmans
2015-04-30 13:54 ` Arnd Bergmann
2015-05-01 6:08 ` Mike Looijmans
2015-05-01 7:01 ` Mike Looijmans
2015-05-07 11:18 ` Mike Looijmans
2015-05-07 11:56 ` Arnd Bergmann
2015-05-07 13:21 ` Daniel Drake
2015-05-07 13:31 ` Mike Looijmans
2015-05-07 14:08 ` Mike Looijmans
2015-05-07 14:30 ` Russell King - ARM Linux
2015-05-08 5:55 ` Mike Looijmans [this message]
2015-05-08 7:54 ` Arnd Bergmann
2015-05-08 8:31 ` Mike Looijmans
2015-05-08 13:19 ` Arnd Bergmann
2015-05-08 14:18 ` Mike Looijmans
2015-05-08 14:27 ` Arnd Bergmann
2015-05-08 11:10 ` Russell King - ARM Linux
2015-05-08 12:17 ` Mike Looijmans
2015-04-29 11:09 ` Mike Looijmans
2015-04-29 12:35 ` Arnd Bergmann
2015-04-29 12:52 ` Mike Looijmans
2015-04-29 12:54 ` Arnd Bergmann
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=554C4FCE.6070802@topic.nl \
--to=mike.looijmans@topic$(echo .)nl \
--cc=linux-arm-kernel@lists$(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