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

From: Viresh Kumar <viresh.kumar@linaro.org>
To: Linus Walleij <linus.walleij@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>,
	"Alex Bennée" <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>
Subject: Re: [PATCH V3 1/3] gpio: Add virtio-gpio driver
Date: Wed, 16 Jun 2021 07:15:20 +0530	[thread overview]
Message-ID: <20210616014520.dkkbdamgelptl3un@vireshk-i7> (raw)
In-Reply-To: <CACRpkdbyybnxP9p3Jh2TbJMhK+kfLyed-YyrZq9JbOqu1csmww@mail.gmail.com>

On 15-06-21, 22:03, Linus Walleij wrote:
> 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

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