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

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Message-ID: <52be16427347cefff9238902146df110@dk-develop.de>
Date:   Thu, 17 Aug 2017 16:14:23 +0200
From:   Danilo Krummrich <danilokrummrich@...develop.de>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
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 2017-08-17 15:01, Russell King - ARM Linux wrote:
> 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.
> 
Thanks again, I'm aware of that. As in my case the code could be called 
from
atomic context disable_irq() is not an option.

My main point is if it can be assumed that after disable_irq_nosync() 
returns
it is guaranteed, by convention, that the hardware was hit. But I really 
would
think so.