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[72.189.64.225]) by smtp.gmail.com with ESMTPSA id g5sm3325325qto.39.2020.10.18.09.58.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 18 Oct 2020 09:58:24 -0700 (PDT) Date: Sun, 18 Oct 2020 12:58:22 -0400 From: William Breathitt Gray To: David Lechner Cc: jic23@kernel.org, kamel.bouhara@bootlin.com, gwendal@chromium.org, alexandre.belloni@bootlin.com, linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, 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 Subject: Re: [PATCH v5 3/5] counter: Add character device interface Message-ID: <20201018165822.GE231549@shinobu> References: <00be1fccc672c5207f3b04fe4cc09c29e22641f4.1601170670.git.vilhelm.gray@gmail.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="idY8LE8SD6/8DnRI" Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --idY8LE8SD6/8DnRI Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Oct 14, 2020 at 05:40:44PM -0500, David Lechner wrote: > On 9/26/20 9:18 PM, William Breathitt Gray wrote: > > +static ssize_t counter_chrdev_read(struct file *filp, char __user *buf, > > + size_t len, loff_t *f_ps) > > +{ > > + struct counter_device *const counter =3D filp->private_data; > > + int err; > > + unsigned long flags; > > + unsigned int copied; > > + > > + if (len < sizeof(struct counter_event)) > > + return -EINVAL; > > + > > + do { > > + if (kfifo_is_empty(&counter->events)) { > > + if (filp->f_flags & O_NONBLOCK) > > + return -EAGAIN; > > + > > + err =3D wait_event_interruptible(counter->events_wait, > > + !kfifo_is_empty(&counter->events)); > > + if (err) > > + return err; > > + } > > + > > + raw_spin_lock_irqsave(&counter->events_lock, flags); > > + err =3D kfifo_to_user(&counter->events, buf, len, &copied); > > + raw_spin_unlock_irqrestore(&counter->events_lock, flags); > > + if (err) > > + return err; > > + } while (!copied); > > + > > + return copied; > > +} >=20 > All other uses of kfifo_to_user() I saw use a mutex instead of spin > lock. I don't see a reason for disabling interrupts here. The Counter character device interface is special in this case because counter->events could be accessed from an interrupt context. This is possible if counter_push_event() is called for an interrupt (as is the case for the 104_quad_8 driver). In this case, we can't use mutex because we can't sleep in an interrupt context, so our only option is to use spin lock. 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