[PATCH v2 4/7] dma: of: introduce of_dma_is_coherent() helper

[Rob Herring]

On Fri, Feb 28, 2014 at 3:39 AM, Arnd Bergmann <arnd at arndb.de> wrote:
> On Thursday 27 February 2014 16:17:49 Santosh Shilimkar wrote:
>> +
>> +/**
>> + * of_dma_is_coherent - Check if device is coherent
>> + * @np:        device node
>> + *
>> + * It returns true if "dma-coherent" property was found
>> + * for this device in DT.
>> + */
>> +bool of_dma_is_coherent(struct device_node *np)
>> +{
>> +       struct device_node *node = np;
>> +
>> +       while (node) {
>> +               if (of_property_read_bool(node, "dma-coherent")) {
>> +                       of_node_put(node);
>> +                       return true;
>> +               }
>> +               node = of_get_next_parent(node);
>> +       }
>> +       return false;
>> +}
>> +EXPORT_SYMBOL_GPL(of_dma_is_coherent);
>>
>
> This won't work on architectures that are always coherent and
> did not need 'dma-coherent' properties before, such as IBM
> Power servers.
>
> That said, I think the property makes sense, and we already have
> platforms using it (highbank is the one I'm aware of).
>
> We probably need ways to override this function in both ways:
> "always coherent" (powerpc, x86), and "never coherent" (keystone
> without LPAE) from platform code, and it would be nice to put
> either option into DT in a global location as well. We may have
> to go through a few iterations of this patch to get the best
> algorithm, but I think the interface is good at least.

I know Will D was not a fan of this property. Primarily I believe
because you may need to describe more than just a boolean in more
complex bus topologies.

Effectively, highbank is always coherent. It was only PCI that is
non-coherent, but I can safely say PCI will never be enabled at this
point. There are no designs with PCI beyond 1 or 2 validation boards
(total boards, not designs), and getting PCI to work was quite hacky
due to only a 1MB window. The other masters are programmable, but only
the coherent path is used as the non-coherent path actually has some
issues. I had expected the opposite believing the ACP port would
actually have issues which is also why I made it configurable.

Rob