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

[Danilo Krummrich]

Hi Linus,

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.)
Therefore I would like to keep this barrier and replace it with 
smp_wmb() if you
are fine with that.

Regards,
Danilo