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

Ley Foon Tan
2014-07-30T03:42:28Z

On Thu, Jul 24, 2014 at 8:05 PM, Arnd Bergmann <arnd@arndb.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