On Friday 28 February 2014 10:14 AM, Rob Herring wrote:
On Fri, Feb 28, 2014 at 3:39 AM, Arnd Bergmann [off-list ref] wrote:
quoted
On Thursday 27 February 2014 16:17:49 Santosh Shilimkar wrote:
quoted
+
+/**
+ * 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.
I also like it to be configurable per device since you can handle
the broken masters if any. On Keystone we have one of the master
broken(non-PCI) and this helps to take care of that case.
Regards,
Santosh