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[113.37.72.20]) by smtp.gmail.com with ESMTPSA id a19sm6965305pff.186.2021.02.25.16.03.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 Feb 2021 16:03:56 -0800 (PST) Date: Fri, 26 Feb 2021 09:03:48 +0900 From: William Breathitt Gray To: Jonathan Cameron Cc: kernel@pengutronix.de, linux-stm32@st-md-mailman.stormreply.com, a.fatoum@pengutronix.de, kamel.bouhara@bootlin.com, gwendal@chromium.org, alexandre.belloni@bootlin.com, david@lechnology.com, linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com, patrick.havelange@essensium.com, fabrice.gasnier@st.com, mcoquelin.stm32@gmail.com, alexandre.torgue@st.com, o.rempel@pengutronix.de Subject: Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute Message-ID: References: <013b2b8682ddc3c85038083e6d5567696b6254b3.1613131238.git.vilhelm.gray@gmail.com> <20210214181146.66d43da7@archlinux> <20210221155140.3e1ef13c@archlinux> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="76Dn/TuCDFUTSoIf" Content-Disposition: inline In-Reply-To: <20210221155140.3e1ef13c@archlinux> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --76Dn/TuCDFUTSoIf Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sun, Feb 21, 2021 at 03:51:40PM +0000, Jonathan Cameron wrote: > On Thu, 18 Feb 2021 19:32:16 +0900 > William Breathitt Gray wrote: >=20 > > On Sun, Feb 14, 2021 at 06:11:46PM +0000, Jonathan Cameron wrote: > > > On Fri, 12 Feb 2021 21:13:44 +0900 > > > William Breathitt Gray wrote: > > > =20 > > > > The events_queue_size sysfs attribute provides a way for users to > > > > dynamically configure the Counter events queue size for the Counter > > > > character device interface. The size is in number of struct > > > > counter_event data structures. The number of elements will be round= ed-up > > > > to a power of 2 due to a requirement of the kfifo_alloc function ca= lled > > > > during reallocation of the queue. > > > >=20 > > > > Cc: Oleksij Rempel > > > > Signed-off-by: William Breathitt Gray > > > > --- > > > > Documentation/ABI/testing/sysfs-bus-counter | 8 +++++++ > > > > drivers/counter/counter-chrdev.c | 23 +++++++++++++++++= ++ > > > > drivers/counter/counter-chrdev.h | 2 ++ > > > > drivers/counter/counter-sysfs.c | 25 +++++++++++++++++= ++++ > > > > 4 files changed, 58 insertions(+) > > > >=20 > > > > diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Document= ation/ABI/testing/sysfs-bus-counter > > > > index 847e96f19d19..f6cb2a8b08a7 100644 > > > > --- a/Documentation/ABI/testing/sysfs-bus-counter > > > > +++ b/Documentation/ABI/testing/sysfs-bus-counter > > > > @@ -212,6 +212,14 @@ Description: > > > > both edges: > > > > Any state transition. > > > > =20 > > > > +What: /sys/bus/counter/devices/counterX/events_queue_size > > > > +KernelVersion: 5.13 > > > > +Contact: linux-iio@vger.kernel.org > > > > +Description: > > > > + Size of the Counter events queue in number of struct > > > > + counter_event data structures. The number of elements will be > > > > + rounded-up to a power of 2. > > > > + > > > > What: /sys/bus/counter/devices/counterX/name > > > > KernelVersion: 5.2 > > > > Contact: linux-iio@vger.kernel.org > > > > diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/cou= nter-chrdev.c > > > > index 16f02df7f73d..53eea894e13f 100644 > > > > --- a/drivers/counter/counter-chrdev.c > > > > +++ b/drivers/counter/counter-chrdev.c > > > > @@ -375,6 +375,29 @@ void counter_chrdev_remove(struct counter_devi= ce *const counter) > > > > cdev_del(&counter->chrdev); > > > > } > > > > =20 > > > > +int counter_chrdev_realloc_queue(struct counter_device *const coun= ter, > > > > + size_t queue_size) > > > > +{ > > > > + int err; > > > > + DECLARE_KFIFO_PTR(events, struct counter_event); > > > > + unsigned long flags; > > > > + > > > > + /* Allocate new events queue */ > > > > + err =3D kfifo_alloc(&events, queue_size, GFP_ATOMIC); =20 > > >=20 > > > Is there any potential for losing events? =20 > >=20 > > We take the events_list_lock down below so we're safe against missing an > > event, but past events currently unread in the queue will be lost. > >=20 > > Shortening the size of the queue is inherently a destructive process if > > we have more events in the current queue than can fit in the new queue. > > Because we a liable to lose some events in such a case, I think it's > > best to keep the behavior of this reallocation consistent and have it > > provide a fresh empty queue every time, as opposed to sometimes dropping > > events and sometimes not. > >=20 > > I also suspect an actual user would be setting the size of their queue > > to the required amount before they begin watching events, rather than > > adjusting it sporadically during a live operation. > > >=20 > Absolutely agree. As such I wonder if you are better off enforcing this > behaviour? If the cdev is open for reading, don't allow the fifo to be > resized.=20 >=20 > Jonathan I can't really think of a good reason not to, so let's enforce it: if the cdev is open, then we'll return an EINVAL if the user attempts to resize the queue. What is a good way to check for this condition? Should I just call kref_read() and see if it's greater than 1? For example, in counter_chrdev_realloc_queue(): if (kref_read(&counter->dev.kobj.kref) > 1) return -EINVAL; William Breathitt Gray --76Dn/TuCDFUTSoIf Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEEk5I4PDJ2w1cDf/bghvpINdm7VJIFAmA4OswACgkQhvpINdm7 VJI4LBAApcdc9OT2tNICqKgjdL9fnN1UvMR4Mymto4r9OKzes/KKt7jFjbmqsaPU IpFVtpG2apdrK7LcCnvxnSAxCwOIkM1r0TOUQjiiWKggpibCOJCXcI1UWu6FTxtX 3FcYUZZMmjFWvijm0LG3I/7UEo+wf3BcatoYSEERpok3IiPhzag+bG1WQhoirpbK 3UdOHyRkYN8kyU4pfFK+T6A2pHO09fFmSJJB2oQ9d90avrJikMkBGQp6/qpIvhPM gMxbYwDxuSG7zCqkIU/GW22VvwhehfGQaoc5u/WlCks271YWPNcMyq94479EM7q7 SP5STZczS/Jxc5Wq5HHuFsCQaSGTJ6uFIqjey88gjlkyvJ+Vzznj6OfBBRFFPPPe KL8UUXk7E9AM17u+Zoj8xV3NnBKixWTKcUnazPsM7QZSPH3fvLmdUbODFhU20gkf yYLSMZgxvkZSe3jHwaQn92MRCMPgqK2vDUIGVnvAYi+nmPQ/ly4VHwcpRi0iS2Pd p95tFoo2gD8vCwoUm4qm/h+n3WYbQZ44D53fUSAR4yQfdntmCueSFKQ1Cty/03uL BVDbwQs+GfKzPttZoChC2I4oNAyjnqrS861Q0Uz5Go1aqFE5GvfJluSrmsIYNf0u XiuOzzN/i9NOBe4GaeCeaaVQAznfbfat6Ym4nuXMOu4sV5twIJg= =vTG+ -----END PGP SIGNATURE----- --76Dn/TuCDFUTSoIf--