linux-kernel - Re: [PATCH v3 4/8] pinctrl: qcom: sdm845: Provide ACPI support

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Date:   Mon, 10 Jun 2019 12:20:30 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     alokc@...eaurora.org, Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        wsa+renesas@...g-engineering.com,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>, balbi@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeffrey Hugo <jlhugo@...il.com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-usb <linux-usb@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 4/8] pinctrl: qcom: sdm845: Provide ACPI support

On Mon, 10 Jun 2019 at 11:22, Lee Jones <lee.jones@...aro.org> wrote:
>
> On Mon, 10 Jun 2019, Ard Biesheuvel wrote:
>
> > On Mon, 10 Jun 2019 at 10:55, Lee Jones <lee.jones@...aro.org> wrote:
> > >
> > > On Mon, 10 Jun 2019, Ard Biesheuvel wrote:
> > >
> > > > On Mon, 10 Jun 2019 at 10:42, Lee Jones <lee.jones@...aro.org> wrote:
> > > > >
> > > > > This patch provides basic support for booting with ACPI instead
> > > > > of the currently supported Device Tree.  When doing so there are a
> > > > > couple of differences which we need to taken into consideration.
> > > > >
> > > > > Firstly, the SDM850 ACPI tables omit information pertaining to the
> > > > > 4 reserved GPIOs on the platform.  If Linux attempts to touch/
> > > > > initialise any of these lines, the firmware will restart the
> > > > > platform.
> > > > >
> > > > > Secondly, when booting with ACPI, it is expected that the firmware
> > > > > will set-up things like; Regulators, Clocks, Pin Functions, etc in
> > > > > their ideal configuration.  Thus, the possible Pin Functions
> > > > > available to this platform are not advertised when providing the
> > > > > higher GPIOD/Pinctrl APIs with pin information.
> > > > >
> > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > >
> > > > For the ACPI probing boilerplate:
> > > > Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> > > >
> > > > *However*, I really don't like hardcoding reserved GPIOs like this.
> > > > What guarantee do we have that each and every ACPI system
> > > > incorporating the QCOM0217 device has the exact same list of reserved
> > > > GPIOs?
> > >
> > > This is SDM845 specific, so the chances are reduced.
> >
> > You don't know that.
>
> All the evidence I have to hand tells me that this is the case.  Even
> on very closely related variants Qualcomm uses different H/W blocks
> for GPIO.
>
> > > However, if another SDM845 variant does crop up, also lacking the
> > > "gpios" property, we will have to find another differentiating factor
> > > between them and conduct some matching.  What else can you do with
> > > platforms supporting non-complete/non-forthcoming ACPI tables?
> > >
> >
> > Either we don't touch any pins at all if they are not referenced
> > explicitly anywhere
>
> I guess this would require an API change, which is out of scope of
> this patch-set.  Happy to change this implementation later if the
> subsystem allows for it though.
>
> > or we parse the PEP tables, which seem to cover
> > some of this information (if Bjorn's analysis is correct)
>
> Maybe someone can conduct some further work on this when we start to
> enable or write a driver for the PEP (Windows-compatible System Power
> Management Controller).  The tables for the PEP look pretty complex,
> so this task would be extremely difficult if not impossible without
> Qualcomm's help.  I wouldn't even know how to extrapolate this
> information from the tables.
>
> > (if Bjorn's analysis is correct)
>
> Bjorn is about to provide his Reviewed-by for this implementation.
>

If Bjorn can live with it, then so can I.