linux-kernel - Re: [PATCH v4 3/5] gpio: msc313: MStar MSC313 GPIO driver

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Date:   Fri, 11 Dec 2020 00:49:04 +0900
From:   Daniel Palmer <daniel@...f.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     SoC Team <soc@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh@...nel.org>, Willy Tarreau <w@....eu>
Subject: Re: [PATCH v4 3/5] gpio: msc313: MStar MSC313 GPIO driver

Hi Arnd,

On Thu, 10 Dec 2020 at 23:28, Arnd Bergmann <arnd@...nel.org> wrote:
> > I did think about this and I did this with the clk mux driver I
> > haven't pushed yet. In that case there is a random lump of registers
> > with some muxes mixed into it so I decided to make the lump a syscon
> > and then have a node for each clk mux in the lump and some properties
> > for the muxes within. The driver is certainly less complex but the
> > device tree is pretty unmanageable as there are probably 30 or more
> > muxes.
>
> Right, for clk drivers, the trade-off is often different, it's not
> unusual that they are a bit of a mess and require a separate driver for
> each cheap.

I will try to clean up the additions for the ssd202 (the smp enabled
chip) this weekend and send a series for that.
If it still seems wrong after adding that I will can that series and
refactor this before lumping more on top of it.

> > > It would be helpful here to replace all the readb_relaxed/writeb_relaxed()
> > > with normal readb()/writeb(). Don't use _relaxed() unless there is a strong
> > > reason why you have to do it, and if you do, explain it in a comment what
> > > the reason is.
> >
> > The reason is that readb()/writeb() will invoke the heavy memory
> > barrier even though it's not needed for peripheral registers.
> > I guess it doesn't actually make all that much difference in reality.
>
> Ah, I forgot you had that heavy barrier. It depends a bit on what you
> use the GPIOs for then. For most uses I think the overhead does not
> matter, but if there is any bit-banged I/O it might make a difference.

Bit-banged buses are likely to happen I think as there is a lot of
gpio compared to hardware peripherals.
Anyhow, I'll add a comment for the readb_relaxed()/writeb_relaxed() usage.

Cheers,

Daniel