linux-kernel - Re: [PATCH v2 2/5] gpio: loongson1: Use readl() & writel()

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Date:   Wed, 8 Mar 2023 19:35:45 +0800
From:   Keguang Zhang <keguang.zhang@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Subject: Re: [PATCH v2 2/5] gpio: loongson1: Use readl() & writel()

On Wed, Mar 8, 2023 at 5:42 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Keguang Zhang
> > Sent: 07 March 2023 03:46
> >
> > On Mon, Mar 6, 2023 at 5:30 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> > >
> > > On Thu, Mar 2, 2023 at 1:52 PM Keguang Zhang <keguang.zhang@...il.com> wrote:
> > > >
> > > > This patch replace __raw_readl() & __raw_writel() with readl() & writel().
> > > >
> > >
> > > Please say WHY you're doing this.
> > >
> > readl & writel contain memory barriers which can guarantee access order.
>
> So what...
>
> There is a data dependency between the read and write.
> The read can't be scheduled before the lock is acquired.
> The write can't be scheduled after the lock is released.
>
> So any barriers in readl()/writel() aren't needed.
>
> If they are only compile barriers they'll have no real effect.
> OTOH if the cpu needs actual synchronising instructions (as some
> ppc do) then they will slow things down.
>
Thanks for the explanation.
The intention of this change is to prevent possible order issues.
At present, __raw_readl() & __raw_writel() do work fine.
I will drop this patch in the next version.


>         David
>
> >
> > > Bart
> > >
> > > > Signed-off-by: Keguang Zhang <keguang.zhang@...il.com>
> > > > ---
> > > > V1 -> V2: Split this change to a separate patch
> > > > ---
> > > >  drivers/gpio/gpio-loongson1.c | 8 ++++----
> > > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/gpio/gpio-loongson1.c b/drivers/gpio/gpio-loongson1.c
> > > > index 8862c9ea0d41..b6c11caa3ade 100644
> > > > --- a/drivers/gpio/gpio-loongson1.c
> > > > +++ b/drivers/gpio/gpio-loongson1.c
> > > > @@ -23,8 +23,8 @@ static int ls1x_gpio_request(struct gpio_chip *gc, unsigned int offset)
> > > >         unsigned long flags;
> > > >
> > > >         raw_spin_lock_irqsave(&gc->bgpio_lock, flags);
> > > > -       __raw_writel(__raw_readl(gpio_reg_base + GPIO_CFG) | BIT(offset),
> > > > -                    gpio_reg_base + GPIO_CFG);
> > > > +       writel(readl(gpio_reg_base + GPIO_CFG) | BIT(offset),
> > > > +              gpio_reg_base + GPIO_CFG);
> > > >         raw_spin_unlock_irqrestore(&gc->bgpio_lock, flags);
> > > >
> > > >         return 0;
> > > > @@ -35,8 +35,8 @@ static void ls1x_gpio_free(struct gpio_chip *gc, unsigned int offset)
> > > >         unsigned long flags;
> > > >
> > > >         raw_spin_lock_irqsave(&gc->bgpio_lock, flags);
> > > > -       __raw_writel(__raw_readl(gpio_reg_base + GPIO_CFG) & ~BIT(offset),
> > > > -                    gpio_reg_base + GPIO_CFG);
> > > > +       writel(readl(gpio_reg_base + GPIO_CFG) & ~BIT(offset),
> > > > +              gpio_reg_base + GPIO_CFG);
> > > >         raw_spin_unlock_irqrestore(&gc->bgpio_lock, flags);
> > > >  }
> > > >
> > > > --
> > > > 2.34.1
> > > >
> >
> >
> >
> > --
> > Best regards,
> >
> > Kelvin Cheung
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)



--
Best regards,

Kelvin Cheung