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

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2020/10/14 22:02, Arnd Bergmann wrote:
> On Wed, Oct 14, 2020 at 3:36 PM Leizhen (ThunderTown)
> <thunder.leizhen@huawei.com> wrote:
>> On 2020/10/14 15:38, Arnd Bergmann wrote:
>>> On Wed, Oct 14, 2020 at 5:15 AM Florian Fainelli <f.fainelli@gmail.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@huawei.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.
> 
> The USB hosts here don't use an IOMMU though, right?

Generally, USB devices are accessed through the IOMMU. However, even in this
case, dma-ranges is not necessarily required. There are many examples of this
in arch/arm64/boot/dts/. For example: arch/arm64/boot/dts/arm/juno.dt.yaml.

Not sure, but maybe I found the answer.

vi drivers/of/address.c +457
457                                               Thus we treat the absence of
458          * "ranges" as equivalent to an empty "ranges" property which means
459          * a 1:1 translation at that level.

466          * This quirk also applies for 'dma-ranges' which frequently exist in
467          * child nodes without 'dma-ranges' in the parent nodes. --RobH

475         if (ranges == NULL || rlen == 0) {
476                 offset = of_read_number(addr, na);
477                 memset(addr, 0, pna * 4);
478                 pr_debug("empty ranges; 1:1 translation\n");
479                 goto finish;
480         }

By the way: At first, I thought that these errors was detected by YAML. Now,
I found that it was generated by "make dtbs". That's why it was reported by
any YAML. Thus, the need to fix these errors is even more urgent.


> 
>>> 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.

OK, I got it.

> 
>       Arnd
> 
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