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:
[..]
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?
[...]
+
+ /**
+ * 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
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