Hi Bjorn/Rafael,

Let me redo the patch with enum then. At least, that's more clear to 
everyone.

Thanks,

Suravee

On 10/19/15 21:17, Bjorn Helgaas wrote:

On Tue, Oct 13, 2015 at 06:53:28PM -0500, Suravee Suthikulanit wrote:

quoted

Bjorn / Rafael,

On 10/13/2015 10:52 AM, Suravee Suthikulpanit wrote:

quoted

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

quoted

[..]
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?

Thinking about this again, I still think having one API (which can
tell whether DMA is supported or not, and if so whether it is
coherent or non-coherent) would be the least confusing and least
error prone.

What if we would just have:

     enum dev_dma_type acpi_get_dev_dma_type(struct acpi_device *adev);

where:
     enum dev_dma_type {
         DEV_DMA_NOT_SUPPORTED,
         DEV_DMA_NON_COHERENT,
         DEV_DMA_COHERENT,
     };

This would probably mean that we should modify
drivers/base/property.c to replace:
     bool device_dma_is_coherent()
to:
     enum dev_dma_type device_get_dma_type()

We used to discuss the enum approach in the past
(https://lkml.org/lkml/2015/8/25/868). But we only considered at the
ACPI level at the time. Actually, this should also reflect in the
property.c.

At this point, only drivers/crypto/ccp/ccp-platform.c and
drivers/net/ethernet/amd/xgbe/xgbe-main.c are calling the
device_dma_is_coherent(). So, it should be easy to change this API.

OK, I'm fine with either the enum or Rafael's 0/1/-ENOTSUPP idea.

Bjorn
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