linux-kernel - Re: [PATCH v2 3/5] dt-bindings: mips: brcm: Document brcm,bmips-cbr-reg property

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Date: Wed, 8 May 2024 09:44:17 -0700
From: Florian Fainelli <florian.fainelli@...adcom.com>
To: Rob Herring <robh@...nel.org>, Christian Marangi <ansuelsmth@...il.com>
Cc: Hauke Mehrtens <hauke@...ke-m.de>, Rafał Miłecki
 <zajec5@...il.com>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>,
 Álvaro Fernández Rojas <noltari@...il.com>,
 linux-mips@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, Daniel González Cabanelas
 <dgcbueu@...il.com>
Subject: Re: [PATCH v2 3/5] dt-bindings: mips: brcm: Document
 brcm,bmips-cbr-reg property

On 5/7/24 06:07, Rob Herring wrote:
> On Fri, May 03, 2024 at 11:20:59PM +0200, Christian Marangi wrote:
>> Document brcm,bmips-cbr-reg and brcm,bmips-broken-cbr-reg property.
>>
>> Some SoC suffer from a BUG where read_c0_brcm_cbr() might return 0
>> if called from TP1. The CBR address is always the same on the SoC
>> hence it can be provided in DT to handle broken case where bootloader
>> doesn't init it or SMP where read_c0_brcm_cbr() returns 0 from TP1.
>>
>> Usage of this property is to give an address also in these broken
>> configuration/bootloader.
>>
>> If the SoC/Bootloader ALWAYS provide a broken CBR address the property
>> "brcm,bmips-broken-cbr-reg" can be used to ignore any value already set
>> in the registers for CBR address.
> 
> Why can't these be implied from an SoC specific compatible?

Because some SoCs with the same compatible have it right, and some 
wrong, courtesy of how the various OEMs implemented it.

> 
> It's not a great design where you have to update the DT which should be
> provided from the bootloader in order to work-around bootloader
> issues...

The bootloader was designed without DT in mind, and while CFE had a 
callback mechanism to query environment variables and whatnot, those 
devices were stripped out of it.

> 
>>
>> Signed-off-by: Christian Marangi <ansuelsmth@...il.com>
>> ---
>>   .../devicetree/bindings/mips/brcm/soc.yaml    | 32 +++++++++++++++++++
>>   1 file changed, 32 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/mips/brcm/soc.yaml b/Documentation/devicetree/bindings/mips/brcm/soc.yaml
>> index 975945ca2888..29af8f0db785 100644
>> --- a/Documentation/devicetree/bindings/mips/brcm/soc.yaml
>> +++ b/Documentation/devicetree/bindings/mips/brcm/soc.yaml
>> @@ -55,6 +55,21 @@ properties:
>>            under the "cpus" node.
>>           $ref: /schemas/types.yaml#/definitions/uint32
>>   
>> +      brcm,bmips-broken-cbr-reg:
>> +        description: Declare that the Bootloader init a broken
>> +          CBR address in the registers and the one provided from
>> +          DT should always be used.
> 
> Why wouldn't brcm,bmips-cbr-reg being present indicate to use it?
> 
>> +        type: boolean
>> +
>> +      brcm,bmips-cbr-reg:
>> +        description: Reference address of the CBR.
>> +          Some SoC suffer from a BUG where read_c0_brcm_cbr() might
>> +          return 0 if called from TP1. The CBR address is always the
>> +          same on the SoC hence it can be provided in DT to handle
>> +          broken case where bootloader doesn't initialise it or SMP
>> +          where read_c0_brcm_cbr() returns 0 from TP1.
>> +        $ref: /schemas/types.yaml#/definitions/uint32
> 
> CBR is never defined anywhere in this patch.

The very presence of "brcm,bmips-cbr-reg" property should be enough to 
indicate to the kernel that it should the value provided, rather than 
the value returned from read_c0_brcm_cbr(). That is, I don't think there 
is a need to indicate to the kernel that the CBR value is broken, if you 
provide a new value that is enough of a clue to tel you that.
-- 
Florian


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