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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Tue, 15 Jun 2021 22:03:21 +0200
Message-ID: <CACRpkdbyybnxP9p3Jh2TbJMhK+kfLyed-YyrZq9JbOqu1csmww@mail.gmail.com>
Subject: Re: [PATCH V3 1/3] gpio: Add virtio-gpio driver
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        "Enrico Weigelt, metux IT consult" <info@metux.net>,
        Viresh Kumar <vireshk@kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Bill Mills <bill.mills@linaro.org>,
        =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>,
        stratos-dev@op-lists.linaro.org,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        "Stefano Garzarella --cc virtualization @ lists . linux-foundation . org" 
        <sgarzare@redhat.com>, virtualization@lists.linux-foundation.org,
        Alistair Strachan <astrachan@google.com>,
        Wolfram Sang <wsa+renesas@sang-engineering.com>
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On Tue, Jun 15, 2021 at 1:15 PM Viresh Kumar <viresh.kumar@linaro.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

Yours,
Linus Walleij