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

From: Roger Quadros
Date: Wed Feb 19 2020 - 10:41:14 EST

On 19/02/2020 17:25, Robin Murphy wrote:
On 19/02/2020 2:29 pm, Roger Quadros wrote:

On 18/02/2020 19:22, Rob Herring wrote:
On Tue, Feb 18, 2020 at 2:28 AM Roger Quadros <rogerq@xxxxxx> wrote:


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 @@
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ sata: sata@4a141100 {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ compatible = "snps,dwc-ahci";
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ reg = <0x4a140000 0x1100>, <0x4a141100 0x7>;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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.

ÂFrom Documentation/devicetree/booting-without-of.txt

* DMA Bus master
Optional property:
- dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
ÂÂÂÂÂÂÂÂ (child-bus-address, parent-bus-address, length). Each triplet specified
ÂÂÂÂÂÂÂÂ describes a contiguous DMA address range.
ÂÂÂÂÂÂÂÂ The dma-ranges property is used to describe the direct memory access (DMA)
ÂÂÂÂÂÂÂÂ structure of a memory-mapped bus whose device tree parent can be accessed
ÂÂÂÂÂÂÂÂ from DMA operations originating from the bus. It provides a means of
ÂÂÂÂÂÂÂÂ defining a mapping or translation between the physical address space of
ÂÂÂÂÂÂÂÂ the bus and the physical address space of the parent of the bus.
ÂÂÂÂÂÂÂÂ (for more information see the Devicetree Specification)

* DMA Bus child
Optional property:
- dma-ranges: <empty> value. if present - It means that DMA addresses
ÂÂÂÂÂÂÂÂ 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.



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