linux-kernel - Re: [PATCH 3/4] memory: brcmstb

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Date:   Wed, 6 Dec 2023 08:18:02 -0800
From:   Florian Fainelli <florian.fainelli@...adcom.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Markus Mayer <mmayer@...adcom.com>,
        Rob Herring <robh+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>
Cc:     Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
        Device Tree Mailing List <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] memory: brcmstb_dpfe: support DPFE API v4



On 12/6/2023 3:10 AM, Krzysztof Kozlowski wrote:
> On 05/12/2023 19:47, Markus Mayer wrote:
>> Add support for version 4 of the DPFE API. This new version is largely
>> identical to version 3. The main difference is that all commands now
>> take the MHS version number as the first argument. Any other arguments
>> have been pushed down by one (i.e. what used to be arg0 in v3 is arg1 in
>> v4).
>>
>> Signed-off-by: Markus Mayer <mmayer@...adcom.com>
>> ---
> 
> ...
> 
>> +
>>   static const char *get_error_text(unsigned int i)
>>   {
>>   	static const char * const error_text[] = {
>> @@ -929,8 +954,12 @@ static const struct of_device_id brcmstb_dpfe_of_match[] = {
>>   	{ .compatible = "brcm,dpfe-cpu-v1", .data = &dpfe_api_old_v2 },
>>   	{ .compatible = "brcm,dpfe-cpu-v2", .data = &dpfe_api_new_v2 },
>>   	{ .compatible = "brcm,dpfe-cpu-v3", .data = &dpfe_api_v3 },
>> +	{ .compatible = "brcm,dpfe-cpu-v4", .data = &dpfe_api_v4 },
>>   
> 
> No, use SoC specific compatible.

This is not that simple because for a given SoC, the API implemented by 
the firmware can change, in fact it has changed over the lifetime of a 
given SoC as firmware updates get rolled out. Arguably the dialect 
spoken by the firmware should not have changed and we told the firmware 
team about that but it basically went nowhere and here we are.

The Device Tree gets populated by the boot loader which figures out 
which API is spoken and places one of those compatible strings 
accordingly for the kernel to avoid having to do any sort of run-time 
detection which is slow and completely unnecessary when we can simply 
tell it ahead of time what to use.
-- 
Florian

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