linux-kernel - Re: [PATCH v2 1/2] arm64: dts: broadcom: clear the warnings caused by empty dma-ranges

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Date:   Fri, 16 Oct 2020 09:48:15 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        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 v2 1/2] arm64: dts: broadcom: clear the warnings caused by
 empty dma-ranges

On 10/16/20 4:01 AM, Arnd Bergmann wrote:
> On Fri, Oct 16, 2020 at 11:09 AM Zhen Lei <thunder.leizhen@...wei.com> wrote:
>>
>> The scripts/dtc/checks.c requires that the node have empty "dma-ranges"
>> property must have the same "#address-cells" and "#size-cells" values as
>> the parent node. Otherwise, the following warnings is reported:
>>
>> 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)
>>
>> Arnd Bergmann figured out why it's necessary:
>> 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.
>>
>> Suggested-by: Arnd Bergmann <arnd@...db.de>
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> 
> Acked-by: Arnd Bergmann <arnd@...db.de>
> 
> I see that at least the 'bcd' and 'xhci' devices in fact try to
> use 64-bit DMA. It would be good to test this on actual
> hardware to ensure that it works correctly when this is enabled.
> 
> Ideally avoiding the swiotlb bounce buffering should only
> make it faster here, but there are many chips on which
> 64-bit DMA is broken in some form.

Is this change really an improvement though? This 'usb' pseudo bus node
could just keep being defined with #address-cells = <1> and #size-cells
= <1> so as to satisfy the 'reg' definition however we could just adjust
dma-ranges to indicate full 64-bit addressing capability. Would not that
work?
-- 
Florian