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

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Date:   Thu, 10 Aug 2017 16:38:10 +0200
From:   Danilo Krummrich <danilokrummrich@...develop.de>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     linux-kernel@...r.kernel.org,
        Linux Input <linux-input@...r.kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH] serio: PS2 gpio bit banging driver for the serio bus

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