Re: [PATCH] serio: PS2 gpio bit banging driver for the serio bus

[Danilo Krummrich]

On 2017-08-11 11:16, Linus Walleij wrote:
> On Thu, Aug 10, 2017 at 4:38 PM, Danilo Krummrich
> <danilokrummrich@xxxxxxxxxxxxx> wrote:
> > On 2017-08-07 18:22, Danilo Krummrich wrote:
> > > > > +static int ps2_gpio_write(struct serio *serio, unsigned char val)
> > > > > +{
> > > > > +       struct ps2_gpio_data *drvdata = serio->port_data;
> > > > > +
> > > > > +       drvdata->mode = PS2_MODE_TX;
> > > > > +       drvdata->tx_byte = val;
> > > > > +       /* Make sure ISR running on other CPU notice changes. */
> > > > > +       barrier();
> > > >
> > > > This seems overengineered, is this really needed?
> > > >
> > > > If we have races like this, the error is likely elsewhere, and 
> > > > should be
> > > > fixed in the GPIO driver MMIO access or so.
> > > Yes, seems it can be removed. I didn't saw any explicit barriers in 
> > > the
> > > GPIO
> > > driver (I'm testing on bcm2835), but it seems MMIO operations on SMP 
> > > archs
> > > does contain barriers. Not sure if all do. If some do not this 
> > > barrier
> > > might
> > > be needed to ensure ISR on other CPU notice the correct mode and byte 
> > > to
> > > send.
> > I couldn't find any guarantee that the mode and tx_byte change is 
> > implicitly
> > covered by a barrier in this case. E.g. the bcm2835 driver does not 
> > make
> > sure stores are completed before the particular interrupt is enabled, 
> > except by
> > the fact that writel on ARM contains a wmb(). But this is nothing to 
> > rely on.
> > (Please tell me if I miss something.)
>
> writel() should be guaranteeing that the values hit the hardware, wmb() 
> is
> spelled out "write memory barrier" I don't see what you're after here.
Sorry for confusing wording. What I actually meant is if writel() is 
guaranteed
to make sure there's no reordering happening with other store 
operations. Of
course, in case of ARM it is sufficient as it contains a wmb. But I 
wasn't aware
that all writel() implementations guarantee this (if needed).
Thanks for clarification.

> If you think writel() doesn't do its job on some platform, then fix 
> writel()
> on that platform.
>
> We can't randomly sprinkle things like this all over the kernel it 
> makes
> no sense.
>
> > Therefore I would like to keep this barrier and replace it with 
> > smp_wmb() if
> > you are fine with that.
>
> I do not think this is proper.
As you explained writel() should guarantee no reordering with other 
store operations
(like drvdata->mode = PS2_MODE_TX in my case) is happening, I totally 
agree and will
fix this.

> Yours,
> Linus Walleij

Thanks,
Danilo