[PATCH 3/7] of: introduce of_dma_is_coherent() helper

[Rob Herring]

On Thu, Mar 6, 2014 at 9:44 PM, Santosh Shilimkar
<santosh.shilimkar at ti.com> wrote:
> On Friday 07 March 2014 11:13 AM, Rob Herring wrote:
>> On Thu, Mar 6, 2014 at 3:19 AM, Santosh Shilimkar
>> <santosh.shilimkar at ti.com> wrote:
>>> The of_dma_is_coherent() helper parses the given DT device
>>> node to see if the "dma-coherent" property is supported and
>>> returns true or false accordingly.
>>>
>>> For the architectures which are fully dma coherent and don't need per device
>>> property, it can enable CONFIG_ARCH_IS_DMA_COHERENT config option which
>>> enables DMA coherent for all devices by default.
>>
>> This worries me. I killed off arch_is_coherent() for arm. Now we're
>> adding something back. Also, we already have HAVE_GENERIC_DMA_COHERENT
>> which is different, but the names will be confusing. MIPS also has
>> DMA_NONCOHERENT.
>>
> Thanks for comments Rob. I will address them in next version.
> Specifically about ARCH_IS_DMA_COHERENT, I wasn't very comfortable either
> while adding it. But as Arnd mentioned, there is a need to have a way
> for the arch's which are fully coherent to use coherent ops by default.
>
> Am not sure whats the best way to have such support without imposing
> any special updates on such arches.

Thinking about this some more, if the arch is always coherent or
always non-coherent, then the default ops are always fine. In that
case set_arch_dma_coherent_ops is always a nop and of_dma_is_coherent
is a don't care.

Rob