linux-kernel - Re: [PATCH 02/11] nvmem: qfprom: Mark core clk as optional

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Date:   Wed, 30 Aug 2023 07:57:54 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Luca Weiss <luca.weiss@...rphone.com>
Cc:     cros-qcom-dts-watchers@...omium.org,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        ~postmarketos/upstreaming@...ts.sr.ht, phone-devel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH 02/11] nvmem: qfprom: Mark core clk as optional

Hi,

On Wed, Aug 30, 2023 at 7:43 AM Luca Weiss <luca.weiss@...rphone.com> wrote:
>
> On Wed Aug 30, 2023 at 4:30 PM CEST, Doug Anderson wrote:
> > Hi,
> >
> > On Wed, Aug 30, 2023 at 2:58 AM Luca Weiss <luca.weiss@...rphone.com> wrote:
> > >
> > > On some platforms like sc7280 on non-ChromeOS devices the core clock
> > > cannot be touched by Linux so we cannot provide it. Mark it as optional
> > > as accessing qfprom works without it.
> > >
> > > Signed-off-by: Luca Weiss <luca.weiss@...rphone.com>
> > > ---
> > >  drivers/nvmem/qfprom.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > Are you actually testing burning fuses from the OS, or are you just
> > using the nvmem in "read-only" mode? From comments in the bindings, if
> > you're trying to burn the fuses then the clock is required. If things
> > are in read-only mode then the clock isn't required.
>
> Hi Doug,
>
> I definitely don't plan on burning any fuses on this phone. Not even
> sure that's allowed by the TZ / boot stack.
>
> >
> > When I compare to the driver, it seems like the driver assumes that if
> > more than one memory region is provided then you must be supporting
> > burning fuses. The bindings agree that having 4 memory regions
> > specified means that the nvmem supports burning and 1 memory region
> > specified means read-only. The extra 3 memory regions in the nvmem are
> > all about fuse burning, I believe.
> >
> > So maybe the right fix here is to just change your dts to specify one
> > memory region?
>
> I got feedback from Konrad that this here would be the preferred
> approach compared to having a different dts for ChromeOS vs non-ChromeOS
> devices. I don't feel strongly to either, for me it's also okay to
> remove the extra memory regions and only have the main one used on
> regular qcom devices.
>
> Let me know what you think.

I don't hate the idea of leaving the extra memory regions in the dts.
They do describe the hardware, after all, even if the main OS can't
actually access those memory regions. ...though the same could also be
said about the clock you've removed. Said another way: if you want to
fully describe the hardware then the dts should have the extra memory
regions and the clock. If you are OK w/ just describing the hardware
in the way that the OS has access to then the dts should not have the
extra memory regions and not have the clock. Does that sound right?

If somehow you do end up with something like your patch, though,
you're still missing a bit. Specifically, you don't want to "enable
writing" a few lines below if you didn't get the clock, right?

-Doug