linux-kernel - Re: [PATCH v8 3/4] gpiolib: add irq_not_threaded flag to gpio

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Message-ID: <CAE1zot+FL54r3a6-qJwDti3D2dGCSwN=a0g0ki28cy1ToXgSfg@mail.gmail.com>
Date:	Mon, 20 Oct 2014 13:19:12 +0300
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Wolfram Sang <wsa@...-dreams.de>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Arnd Bergmann <arnd@...db.de>, Johan Hovold <johan@...nel.org>,
	Daniel Baluta <daniel.baluta@...el.com>,
	Laurentiu Palcu <laurentiu.palcu@...el.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH v8 3/4] gpiolib: add irq_not_threaded flag to gpio_chip

On Mon, Oct 20, 2014 at 8:08 AM, Alexandre Courbot <gnurou@...il.com> wrote:
>
> On Wed, Oct 15, 2014 at 11:48 PM, Octavian Purdila
> <octavian.purdila@...el.com> wrote:
> > Some GPIO chips (e.g. the DLN2 USB adapter) have blocking get/set
> > operation but do not need a threaded irq handler.
>
> Sorry if you already explained this (I have been a little bit late
> with the GPIO reviews recently), but does this optimization bring a
> significant benefit that justifies adding another field in struct
> gpio_chip? If so it would be nice to have it in the commit message. If
> not, do we need this at all?

Hi Alexandre,

In the case DLN2 USB GPIO adapter the GPIO interrupt is generated in
the completion routine of a receive URB, which means that we are in
interrupt context. If a threaded irq is used, we would have to
schedule work instead of running to interrupt handler directly which
is unnecessary and adds latency.

BTW, AFAIC Linus W already merged this patch in his next tree, I am
keeping it in this series because it was not pulled in the mfd-next
tree.

Thanks,
Tavi
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