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

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Message-ID: <20170817130103.GR20805@n2100.armlinux.org.uk>
Date:   Thu, 17 Aug 2017 14:01:03 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Danilo Krummrich <danilokrummrich@...develop.de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org, 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

On Thu, Aug 17, 2017 at 12:51:33PM +0200, Danilo Krummrich wrote:
> That having the correct execution order is not enough on some buses because
> of buffering is really something to be aware of, thanks again for pointing
> this out.

PCI guarantees the order of writes to a device, but there are situations
on SoCs where you can't rely on that - for instance, if the writes go
over different buses to different devices (eg, write to a peripheral
vs write to an interrupt controller.)

Even then, with interrupts delivered by message (eg, MSI) there's
issues.

> So for the scenario I was concerned about I would expect the irqchip driver
> guarantees the write actually hits the the hardware (if necessary read it
> back) before the function (disable_irq_nosync()) returns, is that correct?
> Though, having the need should be very unlikely.

Well, disable_irq_nosync() doesn't guarantee that the interrupt handler
isn't running - a CPU may have just received the interrupt and is just
entering the interrupt handler when disable_irq_nosync() returns.  The
hint is the "nosync" - there's no synchronisation.  If you need to
guarantee that the interrupt handler is not running, disable_irq() does
that.  By implication, however, disable_irq() can not be called from
within the same interrupt handler for the interrupt that is being
disabled.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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