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

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Date:   Wed, 6 Dec 2023 10:48:30 -0800
From:   Markus Mayer <mmayer@...adcom.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Florian Fainelli <florian.fainelli@...adcom.com>,
        Rob Herring <robh+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        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 Wed, 6 Dec 2023 at 09:32, Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 06/12/2023 17:18, Florian Fainelli wrote:
> >
> >
> > 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.
>
> Thanks for providing justification, quite reasonable. A pity that none
> of the commit msgs answered this way.

The real pity is how this API was designed, making all of this
necessary in the first place.

We can definitely spell out more clearly in the commit messages what
is going on and why all of this is needed. I'll pull all the pieces
together from the various responses. As long as there's a way we can
reasonably implement what we need, we'll be happy.

> Best regards,
> Krzysztof