linux-kernel - Re: [PATCH v2 4/4] arm64: dts: allwinner: h6: Use RSB for AXP805 PMIC connection

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Date:   Wed, 6 Jan 2021 19:38:43 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     André Przywara <andre.przywara@....com>,
        Samuel Holland <samuel@...lland.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH v2 4/4] arm64: dts: allwinner: h6: Use RSB for AXP805 PMIC connection

On Wed, Jan 6, 2021 at 7:06 PM Maxime Ripard <maxime@...no.tech> wrote:
>
> On Mon, Jan 04, 2021 at 10:54:19AM +0000, André Przywara wrote:
> > On 03/01/2021 10:00, Samuel Holland wrote:
> > > On boards where the only peripheral connected to PL0/PL1 is an X-Powers
> > > PMIC, configure the connection to use the RSB bus rather than the I2C
> > > bus. Compared to the I2C controller that shares the pins, the RSB
> > > controller allows a higher bus frequency, and it is more CPU-efficient.
> >
> > But is it really necessary to change the DTs for those boards in this
> > way? It means those newer DTs now become incompatible with older
> > kernels, and I don't know if those reasons above really justify this.
> >
> > I understand that we officially don't care about "newer DTs on older
> > kernels", but do we really need to break this deliberately, for no
> > pressing reasons?
> >
> > P.S. I am fine with supporting RSB on H6, and even using it on new DTs,
> > just want to avoid breaking existing ones.
>
> Doing so would also introduce some inconsistencies, one more thing to
> consider during reviews, and would require more testing effort.
>
> I'm not sure that stretching our - already fairly sparse - resources
> thin would be very wise here, especially for something that we don't
> have to do and for a setup that isn't really used that much.

As soon as some software component starts running RSB, (which I assume
is what Samuel is planning to do in Crust?), there's a chance that it
doesn't switch the chip back to I2C. And then Linux won't be able to
access it.

So I'm for keeping things consistent and converting all users to RSB.


ChenYu