Re: [PATCH 1/2] arm64: dts: broadcom: remove an unused property dma-ranges

[Arnd Bergmann]

On Wed, Oct 14, 2020 at 3:36 PM Leizhen (ThunderTown)
<thunder.leizhen@xxxxxxxxxx> wrote:
> On 2020/10/14 15:38, Arnd Bergmann wrote:
> > On Wed, Oct 14, 2020 at 5:15 AM Florian Fainelli <f.fainelli@xxxxxxxxx> wrote:
> >> On 10/12/2020 11:06 PM, Zhen Lei wrote:
> >>> stingray-usb.dtsi is finally included by three dts files:
> >>> bcm958802a802x.dts, bcm958742k.dts and bcm958742t.dts. I searched all
> >>> these three entire expanded dts files, and each of them contains only one
> >>> dma-ranges. No conversion range is specified, so it cannot work properly.
> >>> I think this property "dma-ranges" is added by mistake, just remove it.
> >>> Otherwise, the following error will be reported when any YAML detection
> >>> is performed on arm64.
> >>>
> >>> arch/arm64/boot/dts/broadcom/stingray/stingray-usb.dtsi:7.3-14: Warning \
> >>> (dma_ranges_format): /usb:dma-ranges: empty "dma-ranges" property but \
> >>> its #address-cells (1) differs from / (2)
> >>> arch/arm64/boot/dts/broadcom/stingray/stingray-usb.dtsi:7.3-14: Warning \
> >>> (dma_ranges_format): /usb:dma-ranges: empty "dma-ranges" property but \
> >>> its #size-cells (1) differs from / (2)
> >>>
> >>> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> >>
> >> This looks fine to me, Scott, Ray do you want to Ack this patch before I
> >> take it?
> >
> > Does it mean that there are no devices on this bus that can do DMA?
> >
> > Usually there should be a dma-ranges property to identify that DMA
> > is possible and what the limits are, though we have failed to enforce
> > that.
>
> Documentation/devicetree/bindings/iommu/iommu.txt +79
> When an "iommus" property is specified in a device tree node, the IOMMU will
> be used for address translation. If a "dma-ranges" property exists in the
> device's parent node it will be ignored. An exception to this rule is if the
> referenced IOMMU is disabled, in which case the "dma-ranges" property of the
> parent shall take effect.
>
> The dma-ranges is only required by IOMMU disabled case. And should exist in
> the parent node of IOMMU device. But this deleted dma-ranges is under the usb
> bus node.

The USB hosts here don't use an IOMMU though, right?

> > Also note that the #address-cells=<1> means that any device under
> > this bus is assumed to only support 32-bit addressing, and DMA will
> > have to go through a slow swiotlb in the absence of an IOMMU.
>
> The dma_alloc_coherent() will allocate memory with GFP_DMA32 flag and
> try the 0-4G first. The reserved swiotlb buffer memory is used only
> when the allocation failed.

The swiotlb is primarily about the streaming mappings with dma_map_*(),
which has to copy all data sent to the device. dma_alloc_coherent()
is a rare operation and less impacted by DMA limitations.

      Arnd