Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency

From: Suravee Suthikulanit
Date: Thu Apr 30 2015 - 19:40:11 EST

On 4/30/2015 3:23 AM, Arnd Bergmann wrote:
On Wednesday 29 April 2015 16:53:10 Suravee Suthikulpanit wrote:
On 4/29/15 11:25, Arnd Bergmann wrote:
On Wednesday 29 April 2015 08:44:09 Suravee Suthikulpanit wrote:
As for the case where _CCA=0, I think the ACPI driver should essentially
communicate the information as HW is non-coherent as described in the
spec, and should be calling arch_setup_dma_ops(dev, false). It is true
that this in probably less-likely for the ARM64 server platforms.
However, I would think that the ACPI driver should not be making such

Can you add a description to the ACPI spec then to describe in detail what
"non-coherent" is supposed to mean, and which action the OS is supposed to
take when accessing data from device or CPU?

I believe Will has already provided this, and we have already discussed this on separate emails in this thread.

On a related note, I'm not sure how to handle different DMA masks here.
arch_setup_dma_ops() gets passed a size (and offset) argument, which should
match the DMA mask, but I don't know if there is a way to find out the
size from ACPI. Should we assume it's always 64-bit DMA capable?

Looking at the ACPI spec, it does have the _DMA object. IIUC, this can
be used to describe DMA properties of a particular bus.

Method(_DMA, ResourceTemplate()
PosDecode, // _DEC
MinFixed, // _MIF
MaxFixed, // _MAF
Prefetchable, // _MEM
ReadWrite, // _RW
0, // _GRA
0, // _MIN
0x1fffffff, // _MAX
0x200000000, // _TRA
0x20000000, // _LEN
, , ,

I am not sure if this is an appropriate use for this object, but this
seems to be similar to the dma-ranges property for OF, and probably can
be used to specify baseaddr and size information when calling

Yes, that seems like a good idea. What is the expected behavior when that
object is absent? Do we assume that the parent device is not DMA capable?

From the spec:
If the _DMA object is not present for a bus device, the OS assumes that any address placed on a bus by a child device will be decoded either by a device on the bus or by the bus itself, (in other words, all address ranges can be used for DMA).

The issue is, since this is optional, I don't know which FW often providing this info.

Is this sufficient to describe the case where a device can only do DMA
to a specific address range that is not at bus address zero but that maps
to the beginning of physical RAM?

I believe that's the _MIN (Minimum Base Address) is for.

For legacy reasons, the default mask is probably best left at 32-bit,
but drivers are expected to call dma_set_mask() if they can do 64-bit DMA,
and that should fail based on the information provided by the platform
if the bus is not capable of doing that.

However, on ARM64 the dma_base and size parameter for
arch_setup_dma_ops() is currently not used, and only coherent flag is

We can hope that we won't need the dma_base setting here, but it's
good to have the option to pass it down if we need it.

Not passing the size is a bug that needs to be fixed ASAP, I believe
a number of folks have run into this, most recently the APM X-Gene
MMC controller

Ok. I'll look at this separately.

We probably should look at this separately. For the moment, we can
probably say that if _CCA object is missing when needed, the ACPI driver
won't set up dma_mask when creating platform_device, which should be
equivalent to saying DMA is not supported.

Please let me know if this is acceptable, and I'll make change in V2

I would still ask that you treat non-coherent to mean "no DMA" until
we have come up with a way to sufficiently describe the kind of
non-coherency in ACPI.


Ok. In V2, when _CCA=0, since we are not aware of ARM64 systems that is working with such assumption with ACPI. I will also default to not calling arch_setup_dma_ops() and fallback to arch-specific default. We can revisit this later once we need to support such case.



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