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

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On 4/29/15 11:25, Arnd Bergmann wrote:
On Wednesday 29 April 2015 08:44:09 Suravee Suthikulpanit wrote:
diff --git a/drivers/acpi/acpi_platform.c b/drivers/acpi/acpi_platform.c
index 4bf7559..a4db208 100644
--- a/drivers/acpi/acpi_platform.c
+++ b/drivers/acpi/acpi_platform.c
@@ -108,9 +108,12 @@ struct platform_device *acpi_create_platform_device(struct acpi_device *adev)
         if (IS_ERR(pdev))
                 dev_err(&adev->dev, "platform device creation failed: %ld\n",
                         PTR_ERR(pdev));
-       else
+       else {
+               arch_setup_dma_ops(&pdev->dev, 0, 0, NULL,
+                                  adev->flags.is_coherent);
                 dev_dbg(&adev->dev, "created platform device %s\n",
                         dev_name(&pdev->dev));
+       }

         kfree(resources);


Looking at this code in more detail, it seems that it unconditionally
sets pdevinfo.dma_mask = DMA_BIT_MASK(32), before calling
arch_setup_dma_ops().

I think that's just the default legacy value assigned when it first create the platform_device from acpi_device.

This assignment should really done inside of arch_setup_dma_ops()
> instead, which means we should implement that
function on all architectures that support ACPI.


For the case where _CCA is missing (or coherency disabled, if you ask
me), we would not call that function.

Actually, I agree for the case of missing _CCA when needed, ACPI driver probably should not make assumption and leave the decision for the default underlying arch-specific default. Basically, it should not be calling arch_setup_dma_ops().

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 assumption.

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()
{
	QWORDMemory(
	ResourceConsumer,
	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 arch_setup_dma_ops().

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.

	Arnd


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

Thanks,

Suravee
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