linux-kernel - Re: [PATCH v2 13/29] nios2: DMA mapping API

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 30 Jul 2014 11:42:27 +0800
From:	Ley Foon Tan <lftan@...era.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Chung-Lin Tang <cltang@...esourcery.com>,
	Richard Kuo <rkuo@...eaurora.org>,
	linux-hexagon@...r.kernel.org, Mark Salter <msalter@...hat.com>,
	Aurelien Jacquiot <a-jacquiot@...com>,
	linux-c6x-dev@...ux-c6x.org
Subject: Re: [PATCH v2 13/29] nios2: DMA mapping API

On Thu, Jul 24, 2014 at 8:05 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> I have spent some times look at other architectures and below is what
>> I found. Please correct me if I am wrong, especially
>> for_device():DMA_FROM_DEVICE.
>>
>> _for_cpu():
>> case DMA_BIDIRECTIONAL:
>> case DMA_FROM_DEVICE:
>>      /* invalidate cache */
>> break;
>> case DMA_TO_DEVICE:
>>    /* do nothing */
>> break;
>
> This seems fine: for a FROM_DEVICE mapping, we have flushed all
> dirty entries during the _for_device or the map operation,
> so if any clean entries are around, they need to be invalidated
> in order to read the data from the device.
>
> for TO_DEVICE, we don't care about the cache, because we are
> going to overwrite the data, and we don't need to do anything.
Okay.

>
>> -------------------------
>> _for_device():
>> case DMA_BIDIRECTIONAL:
>> case DMA_TO_DEVICE:
>>      /* flush and invalidate cache */
>> break;
>> case DMA_FROM_DEVICE:
>>  /* should we invalidate cache or do nothing? */
>> break;
>
> You actually don't need to invalidate the TO_DEVICE mappings
> in both _for_device and _for_cpu. You have to flush them
> in for_device, and you have to invalidate them at least once,
> but don't need to invalidate them again in for_cpu if you have
> done that already in for_device and your CPU does not do any
> speculative prefetches that might populate the dcache.
Nios2 processor doesn't have flush-cache-only instruction. Its flush
instruction will do 2 operations, flush and invalidate cache.


> In case of for_device FROM_DEVICE, you have to invalidate or
> flush the caches to ensure that no dirty cache lines are
> written to memory, but only if your CPU has a write-back
> cache rather than write-through.
Nios2 has a write-back cache. So, will do invalidate cache here.

> For bidirectional mappings, you may have to flush and invalidate.
To confirm, do you mean bidirectional for both for_device and for_cpu?

Thanks.

Regards
Ley Foon
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/