Re: dma_mask limited to 32-bits with OF platform device

[Roger Quadros <rogerq@ti.com>]

On 19/02/2020 17:25, Robin Murphy wrote:
> On 19/02/2020 2:29 pm, Roger Quadros wrote:
>> Rob,
>>
>> On 18/02/2020 19:22, Rob Herring wrote:
>>> On Tue, Feb 18, 2020 at 2:28 AM Roger Quadros <rogerq@ti.com> wrote:
>>>>
>>>> Chrishtoph,
>>>>
>>>> The branch works fine for SATA on DRA7 with CONFIG_LPAE once I
>>>> have the below DT fix.
>>>>
>>>> Do you intend to send these fixes to -stable?
>>>>
>>>> ------------------------- arch/arm/boot/dts/dra7.dtsi -------------------------
>>>> index d78b684e7fca..853ecf3cfb37 100644
>>>> @@ -645,6 +645,8 @@
>>>>                  sata: sata@4a141100 {
>>>>                          compatible = "snps,dwc-ahci";
>>>>                          reg = <0x4a140000 0x1100>, <0x4a141100 0x7>;
>>>> +                       #size-cells = <2>;
>>>> +                       dma-ranges = <0x00000000 0x00000000 0x1 0x00000000>;
>>>
>>> dma-ranges should be in the parent (bus) node, not the device node.
>>
>> I didn't understand why.
>>
>> There are many devices on the parent bus node and all devices might not have the 32-bit DMA limit
>> the SATA controller has.
>>
>> SATA controller is the bus master and the ATA devices are children of the SATA controller.
> 
> But SATA is not a memory-mapped bus - in the context of MMIO, the AHCI is the bus-master device, not a bridge or level of interconnect. The DeviceTree spec[1] clearly defines dma-ranges as an address translation between a "parent bus" and a "child bus".
> 
> If in the worst case this address-limited interconnect really only exists between the AHCI's master interface and everything else in the system, then you'll have to describe it explicitly to meet DT's expectation of a "bus" (e.g. [2]). Yes, it's a bit clunky, but any scheme has its edge cases.

I understand now. Thanks.

> 
>>  From Documentation/devicetree/booting-without-of.txt
>>
>> * DMA Bus master
>> Optional property:
>> - dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
>>          (child-bus-address, parent-bus-address, length). Each triplet specified
>>          describes a contiguous DMA address range.
>>          The dma-ranges property is used to describe the direct memory access (DMA)
>>          structure of a memory-mapped bus whose device tree parent can be accessed
>>          from DMA operations originating from the bus. It provides a means of
>>          defining a mapping or translation between the physical address space of
>>          the bus and the physical address space of the parent of the bus.
>>          (for more information see the Devicetree Specification)
>>
>> * DMA Bus child
>> Optional property:
>> - dma-ranges: <empty> value. if present - It means that DMA addresses
>>          translation has to be enabled for this device.
> 
> Disregarding that this was apparently never in ePAPR, so not grandfathered in to DTSpec, and effectively nobody ever has actually followed it (oh, if only...), note "<empty>" - that still doesn't imply that a *non-empty* dma-ranges would be valid on device nodes.
> 
> Robin.
> 
> [1] https://www.devicetree.org/specifications/
> [2] https://lore.kernel.org/lkml/20181010120737.30300-20-laurentiu.tudor@nxp.com/

-- 
cheers,
-roger
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