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

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Date:   Wed, 14 Oct 2020 21:36:03 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Florian Fainelli <f.fainelli@...il.com>
CC:     Rob Herring <robh+dt@...nel.org>, Ray Jui <rjui@...adcom.com>,
        "Scott Branden" <sbranden@...adcom.com>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] arm64: dts: broadcom: remove an unused property
 dma-ranges



On 2020/10/14 15:38, Arnd Bergmann wrote:
> On Wed, Oct 14, 2020 at 5:15 AM Florian Fainelli <f.fainelli@...il.com> 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@...wei.com>
>>
>> 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.

> 
> 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.

 memory@...00000:
    device_type: ["memory"]
    reg: [[0x0, 0x80000000, 0x0, 0x80000000], [0x8, 0x80000000, 0x1, 0x80000000]]

If this is the actual physical memory size, the #address-cells should be set to <1>.

> 
>       Arnd
> 
> .
>