Bjorn / Rafael,

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

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.

Please let me know your opinions, or if you have other suggestions.

Thanks again,
Suravee