Hi Bjorn,

Thanks for your feedback. And sorry for late response. Some how I didn't 
see this earlier.  Please see my comments below.

On 09/14/2015 09:34 AM, Bjorn Helgaas wrote:

quoted

[..]
So, in order to simplify the function, this patch renames acpi_check_dma()
to acpi_check_dma_coherency() to clearly indicate the purpose of this
function, and only returns an integer where -1 means DMA not supported,
1 means coherent DMA, and 0 means non-coherent DMA.

I think acpi_check_dma_coherency() is better, but only slightly.  It
still doesn't give a hint about the *sense* of the return value.  I
think it'd be easier to read if there were two functions, e.g.,

I have been going back-and-forth between the current version, and the 
two-function-approach in the past. I can definitely go with this route 
if you would prefer. Although, if acpi_dma_is_coherent() == 0, it would 
be ambiguous whether DMA is not supported or non-coherent DMA is 
supported. Then, we would need to call acpi_dma_is_supported() to find 
out. So, that's okay with you?

quoted

[...]
+
+	/**
+	 * Currently, we only support _CCA=1 (i.e. coherent_dma=1)
+	 * This should be equivalent to specifying dma-coherent for
+	 * a device in OF.
+	 *
+	 * For the case when _CCA=0 (i.e. coherent_dma=0 && cca_seen=1),
+	 * we have two choices:
+	 *   1. Do not support and disable DMA.

I know you didn't write this comment, but do we actually *disable* DMA in
the sense of turning off PCI bus mastering or calling an ACPI method that
disables DMA by this device?  I suspect we just don't set up DMA ops and
masks for this device.

Actually, I wrote this comment. When we disable DMA, we basically set 
dma-mask=0 and do not setup DMA ops as you mentioned. We don't actually 
mess with the hardware.

Thanks,
Suravee