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

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Message-ID: <18112862-a42e-95b1-39a3-2e414667f39b@broadcom.com>
Date:   Tue, 12 Jan 2021 10:28:13 -0800
From:   Ray Jui <ray.jui@...adcom.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     Bharat Gooty <bharat.gooty@...adcom.com>,
        devicetree <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Scott Branden <sbranden@...adcom.com>,
        Ray Jui <rjui@...adcom.com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/2] arm64: dts: broadcom: clear the warnings caused by
 empty dma-ranges

Hi Arnd,

On 2020-12-15 7:49 a.m., Arnd Bergmann wrote:
> On Tue, Dec 15, 2020 at 4:41 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>>
>> On 12/15/2020 5:19 AM, Bharat Gooty wrote:
>>> Since the IOMMU is disabled and DMA engine is on 32-bit bus, We can not
>>> give the complete DDR for the USB DMA.
>>> So restricting the usable DAM size to 4GB.
>>
>> Thanks, can you make this a proper patch submission along with a Fixes:
>> tag that is:
>>
>> Fixes: 2013a4b684b6 ("arm64: dts: broadcom: clear the warnings caused by
>> empty dma-ranges")
> 
> Yes, that would be helpful, though I would appreciate a better description
> that explains what is actually going on: is it the device or the bus that
> has the 32-bit limitation, and if it is indeed a bug in the device, why do
> you pretend that this is a 64-bit device on a 32-bit bus instead (this
> could also use a comment in the dts file)?
> 
>         Arnd
> 

Sorry for the delay in reply. Bharat finally got time to do some 
investigation to confirm the following:

These USB controllers indeed can address 64-bit. However, on the bus 
internally, only 40-bits are connected to the interconnect of CCN. As a 
result, the 'dma-ranges' should be modified to address 40-bit in size.

We also have a somewhat related question, is it true that since v5.10, 
defining of 'dma-ranges' on the bus node where its child device node has 
implication of IOMMU usage (through 'iommus' or 'iommu-map') is now 
mandatory? My understanding is that the 'dma-ranges' in this scheme will 
define the IOVA address to system address mapping required by all 
devices on that bus. Please help to confirm if my understanding is correct.

Thanks,

Ray

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