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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.
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