Messages in this thread | | | Date | Thu, 30 Apr 2015 18:39:34 -0500 | From | Suravee Suthikulanit <> | Subject | Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency |
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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 >> assumption. > > 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() >> { >> 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(). > > 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 >> used. > > 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 >> accordingly. > > 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. > > Arnd
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.
Thanks,
Suravee
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