linux-kernel - Re: [PATCH V3 1/3] gpio: Add virtio-gpio driver

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Message-ID: <20210616014520.dkkbdamgelptl3un@vireshk-i7>
Date:   Wed, 16 Jun 2021 07:15:20 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        Viresh Kumar <vireshk@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Bill Mills <bill.mills@...aro.org>,
        Alex Bennée <alex.bennee@...aro.org>,
        stratos-dev@...lists.linaro.org,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "Stefano Garzarella --cc virtualization @ lists . linux-foundation . org" 
        <sgarzare@...hat.com>, virtualization@...ts.linux-foundation.org,
        Alistair Strachan <astrachan@...gle.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>
Subject: Re: [PATCH V3 1/3] gpio: Add virtio-gpio driver

On 15-06-21, 22:03, Linus Walleij wrote:
> On Tue, Jun 15, 2021 at 1:15 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> 
> > I am now wondering how interrupts can be made to work here. Do you
> > have anything in mind for that ?
> 
> The aggregator does not aggregate interrupts only lines for now.
> 
> > GPIO sysfs already supports interrupts, (...)
> 
> I hope that doesn't make you tempted to use sysfs to get interrupts,
> those are awful, they use sysfs_notify_dirent() which means that
> if two IRQs happen in  fast succession you will miss one of them
> and think it was only one.
> 
> The GPIO character device supports low latency events forwarding
> interrupts to userspace including QEMU & similar, timestamps the
> events as close in time as possible to when they actually happen
> (which is necessary for any kind of industrial control) and will never
> miss an event if the hardware can register it. See:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/gpio/gpio-event-mon.c

The current version of backend (I am working on) will be Linux
userspace based, so I would be required to get an interrupt there.

I was planning to use /dev/gpiochipN only for now and not a sysfs
file, so yes it should be fine.

-- 
viresh