Re: [PATCH 7/8] gpiolib: add new ioctl() for monitoring changes in line info
From: Bartosz Golaszewski
Date: Thu Nov 28 2019 - 04:46:01 EST
czw., 28 lis 2019 o 00:23 Kent Gibson <warthog618@xxxxxxxxx> napisaÅ(a):
>
> On Wed, Nov 27, 2019 at 04:50:43PM +0100, Bartosz Golaszewski wrote:
> > År., 27 lis 2019 o 16:24 Kent Gibson <warthog618@xxxxxxxxx> napisaÅ(a):
> > >
> > > On Wed, Nov 27, 2019 at 02:35:09PM +0100, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
> > > >
> > > > Currently there is no way for user-space to be informed about changes
> > > > in status of GPIO lines e.g. when someone else requests the line or its
> > > > config changes. We can only periodically re-read the line-info. This
> > > > is fine for simple one-off user-space tools, but any daemon that provides
> > > > a centralized access to GPIO chips would benefit hugely from an event
> > > > driven line info synchronization.
> > > >
> > > > This patch adds a new ioctl() that allows user-space processes to retrieve
> > > > a file-descriptor for given GPIO lines which can be polled for line status
> > > > change events.
> > > >
> > > > Currently the events are generated on three types of status changes: when
> > > > a line is requested, when it's released and when its config is changed.
> > > > The first two are self-explanatory. For the third one: this will only
> > > > happen when another user-space process calls the new SET_CONFIG ioctl()
> > > > as any changes that can happen from within the kernel (i.e.
> > > > set_transitory() or set_debounce()) are of no interest to user-space.
> > > >
> > > > Signed-off-by: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
> > > > ---
> > > > drivers/gpio/gpiolib.c | 218 ++++++++++++++++++++++++++++++++++++++
> > > > drivers/gpio/gpiolib.h | 1 +
> > > > include/uapi/linux/gpio.h | 36 +++++++
> > > > 3 files changed, 255 insertions(+)
> > > >
> > > > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> > > > index d094b1be334d..be5df4bdf44b 100644
> > > > --- a/drivers/gpio/gpiolib.c
> > > > +++ b/drivers/gpio/gpiolib.c
> > > > @@ -547,6 +547,9 @@ static long linehandle_set_config(struct linehandle_state *lh,
> > > > if (ret)
> > > > return ret;
> > > > }
> > > > +
> > > > + atomic_notifier_call_chain(&desc->gdev->notifier,
> > > > + GPIOLINE_CHANGED_CONFIG, desc);
> > > > }
> > > > return 0;
> > > > }
> > > > @@ -1148,6 +1151,212 @@ static int lineevent_create(struct gpio_device *gdev, void __user *ip)
> > > > return ret;
> > > > }
> > > >
> > > > +struct linechanged_fd_state {
> > > > + struct gpio_device *gdev;
> > > > + struct gpio_desc *descs[GPIOHANDLES_MAX];
> > > > + size_t numdescs;
> > > > + wait_queue_head_t waitqueue;
> > > > + DECLARE_KFIFO(events, struct gpioline_changed, 16);
> > > > + struct mutex lock;
> > > > + struct notifier_block changed_nb;
> > > > +};
> > > > +
> > > > +static int linechanged_fd_release(struct inode *inode, struct file *filep)
> > > > +{
> > > > + struct linechanged_fd_state *lc_state = filep->private_data;
> > > > +
> > > > + atomic_notifier_chain_unregister(&lc_state->gdev->notifier,
> > > > + &lc_state->changed_nb);
> > > > + put_device(&lc_state->gdev->dev);
> > > > + kfree(lc_state);
> > > > +
> > > > + return 0;
> > > > +}
> > > > +
> > > > +static __poll_t linechanged_fd_poll(struct file *filep,
> > > > + struct poll_table_struct *pollt)
> > > > +{
> > > > + struct linechanged_fd_state *lc_state = filep->private_data;
> > > > + __poll_t events = 0;
> > > > +
> > > > + poll_wait(filep, &lc_state->waitqueue, pollt);
> > > > +
> > > > + mutex_lock(&lc_state->lock);
> > > > + if (!kfifo_is_empty(&lc_state->events))
> > > > + events = EPOLLIN | EPOLLRDNORM;
> > > > + mutex_unlock(&lc_state->lock);
> > > > +
> > > > + return events;
> > > > +}
> > > > +
> > > > +static ssize_t linechanged_fd_read(struct file *filep, char __user *buf,
> > > > + size_t count, loff_t *off)
> > > > +{
> > > > + struct linechanged_fd_state *lc_state = filep->private_data;
> > > > + unsigned int copied;
> > > > + int ret;
> > > > +
> > > > + if (count < sizeof(struct gpioline_changed))
> > > > + return -EINVAL;
> > > > +
> > > > + do {
> > > > + mutex_lock(&lc_state->lock);
> > > > + if (kfifo_is_empty(&lc_state->events)) {
> > > > + mutex_unlock(&lc_state->lock);
> > > > + if (filep->f_flags & O_NONBLOCK)
> > > > + return -EAGAIN;
> > > > +
> > > > + ret = wait_event_interruptible(lc_state->waitqueue,
> > > > + !kfifo_is_empty(&lc_state->events));
> > > > + if (ret)
> > > > + return ret;
> > > > + } else {
> > > > + mutex_unlock(&lc_state->lock);
> > > > + }
> > > > +
> > > > + if (mutex_lock_interruptible(&lc_state->lock))
> > > > + return -ERESTARTSYS;
> > > > +
> > > > + ret = kfifo_to_user(&lc_state->events, buf, count, &copied);
> > > > + mutex_unlock(&lc_state->lock);
> > > > + if (ret)
> > > > + return ret;
> > > > +
> > > > + if (copied == 0 && (filep->f_flags & O_NONBLOCK))
> > > > + return -EAGAIN;
> > > > + } while (copied == 0);
> > > > +
> > > > + return copied;
> > > > +}
> > > > +
> > > > +static const struct file_operations linechanged_fd_fileops = {
> > > > + .release = linechanged_fd_release,
> > > > + .owner = THIS_MODULE,
> > > > + .llseek = noop_llseek,
> > > > + .poll = linechanged_fd_poll,
> > > > + .read = linechanged_fd_read,
> > > > +};
> > > > +
> > > > +static struct linechanged_fd_state *
> > > > +to_linechanged_fd_state(struct notifier_block *nb)
> > > > +{
> > > > + return container_of(nb, struct linechanged_fd_state, changed_nb);
> > > > +}
> > > > +
> > > > +static int linechanged_fd_notify(struct notifier_block *nb,
> > > > + unsigned long action, void *data)
> > > > +{
> > > > + struct linechanged_fd_state *lc_state = to_linechanged_fd_state(nb);
> > > > + struct gpio_desc *desc = data;
> > > > + struct gpioline_changed chg;
> > > > + int i, ret;
> > > > +
> > > > + for (i = 0; i < lc_state->numdescs; i++) {
> > > > + /* Are we watching this desc? */
> > > > + if (desc == lc_state->descs[i]) {
> > > > + /* Yes - prepare the event. */
> > > > + memset(&chg, 0, sizeof(chg));
> > > > + chg.line_offset = gpio_chip_hwgpio(desc);
> > > > + chg.event_type = action;
> > > > +
> > > > + mutex_lock(&lc_state->lock);
> > > > + ret = kfifo_put(&lc_state->events, chg);
> > > > + mutex_unlock(&lc_state->lock);
> > > > + if (ret)
> > > > + wake_up_poll(&lc_state->waitqueue, EPOLLIN);
> > > > +
> > > > + return NOTIFY_OK;
> > > > + }
> > > > + }
> > > > +
> > > > + return NOTIFY_DONE;
> > > > +}
> > > > +
> > > > +static int linechanged_fd_create(struct gpio_device *gdev, void __user *ip)
> > > > +{
> > > > + struct gpioline_changed_fd_request changed_req;
> > > > + struct linechanged_fd_state *lc_state;
> > > > + struct gpio_desc *desc;
> > > > + struct file *file;
> > > > + int ret, i, fd;
> > > > + u32 offset;
> > > > +
> > > > + ret = copy_from_user(&changed_req, ip, sizeof(changed_req));
> > > > + if (ret)
> > > > + return -EFAULT;
> > > > +
> > > > + if ((changed_req.num_lines == 0) ||
> > > > + (changed_req.num_lines > GPIOHANDLES_MAX))
> > > > + return -EINVAL;
> > > > +
> > > > + lc_state = kzalloc(sizeof(*lc_state), GFP_KERNEL);
> > > > + if (!lc_state)
> > > > + return -ENOMEM;
> > > > +
> > > > + lc_state->gdev = gdev;
> > > > + get_device(&gdev->dev);
> > > > +
> > > > + for (i = 0; i < changed_req.num_lines; i++) {
> > > > + offset = changed_req.lineoffsets[i];
> > > > + desc = gpiochip_get_desc(gdev->chip, offset);
> > > > + if (IS_ERR(desc)) {
> > > > + ret = PTR_ERR(desc);
> > > > + goto out_free_lc_state;
> > > > + }
> > > > +
> > > > + lc_state->descs[i] = desc;
> > > > + }
> > > > +
> > > > + lc_state->numdescs = changed_req.num_lines;
> > > > +
> > > > + init_waitqueue_head(&lc_state->waitqueue);
> > > > + INIT_KFIFO(lc_state->events);
> > > > + mutex_init(&lc_state->lock);
> > > > +
> > > > + lc_state->changed_nb.notifier_call = linechanged_fd_notify;
> > > > +
> > > > + ret = atomic_notifier_chain_register(&gdev->notifier,
> > > > + &lc_state->changed_nb);
> > > > + if (ret)
> > > > + goto out_free_lc_state;
> > > > +
> > > > + fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
> > > > + if (fd < 0) {
> > > > + ret = fd;
> > > > + goto out_unregister_notifier;
> > > > + }
> > > > +
> > > > + file = anon_inode_getfile("gpio-line-changed-fd",
> > > > + &linechanged_fd_fileops,
> > > > + lc_state, O_RDONLY | O_CLOEXEC);
> > > > + if (IS_ERR(file)) {
> > > > + ret = PTR_ERR(file);
> > > > + goto out_put_unused_fd;
> > > > + }
> > > > +
> > > > + changed_req.fd = fd;
> > > > + ret = copy_to_user(ip, &changed_req, sizeof(changed_req));
> > > > + if (ret) {
> > > > + fput(file);
> > > > + put_unused_fd(fd);
> > > > + return -EFAULT;
> > > > + }
> > > > +
> > > > + fd_install(fd, file);
> > > > +
> > > > + return 0;
> > > > +
> > > > +out_put_unused_fd:
> > > > + put_unused_fd(fd);
> > > > +out_unregister_notifier:
> > > > + atomic_notifier_chain_unregister(&gdev->notifier,
> > > > + &lc_state->changed_nb);
> > > > +out_free_lc_state:
> > > > + kfree(lc_state);
> > > > +
> > > > + return ret;
> > > > +}
> > > > +
> > > > /*
> > > > * gpio_ioctl() - ioctl handler for the GPIO chardev
> > > > */
> > > > @@ -1238,6 +1447,8 @@ static long gpio_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> > > > return linehandle_create(gdev, ip);
> > > > } else if (cmd == GPIO_GET_LINEEVENT_IOCTL) {
> > > > return lineevent_create(gdev, ip);
> > > > + } else if (cmd == GPIO_GET_LINECHANGED_FD_IOCTL) {
> > > > + return linechanged_fd_create(gdev, ip);
> > > > }
> > > > return -EINVAL;
> > > > }
> > > > @@ -1499,6 +1710,8 @@ int gpiochip_add_data_with_key(struct gpio_chip *chip, void *data,
> > > > for (i = 0; i < chip->ngpio; i++)
> > > > gdev->descs[i].gdev = gdev;
> > > >
> > > > + ATOMIC_INIT_NOTIFIER_HEAD(&gdev->notifier);
> > > > +
> > > > #ifdef CONFIG_PINCTRL
> > > > INIT_LIST_HEAD(&gdev->pin_ranges);
> > > > #endif
> > > > @@ -2837,6 +3050,8 @@ static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
> > > > spin_lock_irqsave(&gpio_lock, flags);
> > > > }
> > > > done:
> > > > + atomic_notifier_call_chain(&desc->gdev->notifier,
> > > > + GPIOLINE_CHANGED_REQUESTED, desc);
> > > > spin_unlock_irqrestore(&gpio_lock, flags);
> > > > return ret;
> > > > }
> > > > @@ -2934,6 +3149,8 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
> > > > ret = true;
> > > > }
> > > >
> > > > + atomic_notifier_call_chain(&desc->gdev->notifier,
> > > > + GPIOLINE_CHANGED_RELEASED, desc);
> > > > spin_unlock_irqrestore(&gpio_lock, flags);
> > > > return ret;
> > > > }
> > > > @@ -3097,6 +3314,7 @@ static int gpio_set_bias(struct gpio_chip *chip, struct gpio_desc *desc)
> > > > if (ret != -ENOTSUPP)
> > > > return ret;
> > > > }
> > > > +
> > > > return 0;
> > > > }
> > > >
> > > > diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
> > > > index a1cbeabadc69..8e3969616cfe 100644
> > > > --- a/drivers/gpio/gpiolib.h
> > > > +++ b/drivers/gpio/gpiolib.h
> > > > @@ -54,6 +54,7 @@ struct gpio_device {
> > > > const char *label;
> > > > void *data;
> > > > struct list_head list;
> > > > + struct atomic_notifier_head notifier;
> > > >
> > > > #ifdef CONFIG_PINCTRL
> > > > /*
> > > > diff --git a/include/uapi/linux/gpio.h b/include/uapi/linux/gpio.h
> > > > index 799cf823d493..c61429467dd4 100644
> > > > --- a/include/uapi/linux/gpio.h
> > > > +++ b/include/uapi/linux/gpio.h
> > > > @@ -59,6 +59,40 @@ struct gpioline_info {
> > > > /* Maximum number of requested handles */
> > > > #define GPIOHANDLES_MAX 64
> > > >
> > > > +/**
> > > > + * struct gpioline_changed_fd_request - Information about a linechanged fd
> > > > + * request
> > > > + * @lineoffsets: an array of desired lines, specified by offset index for the
> > > > + * associated GPIO device
> > > > + * @num_lines: number of lines requested in this request, i.e. the number of
> > > > + * valid fields in the above arrays, set to 1 to request a single line
> > > > + * @fd: if successful this field will contain a valid anonymous file handle
> > > > + */
> > > > +struct gpioline_changed_fd_request {
> > > > + __u32 lineoffsets[GPIOHANDLES_MAX];
> > > > + __u32 num_lines;
> > > > + int fd;
> > > > +};
> > > > +
> > >
> > > Wouldn't the most common case be to watch all the lines on a chip?
> > > How about an easy way to do that, say num_lines=0?
> > >
> >
> > IMO this is too implicit - it's literally a magic value. I'd prefer to
> > keep it this way for the same reason I didn't want to have implicit
> > BIAS settings. I prefer the kernel uAPI to be explicit and then we can
> > wrap it in simple helpers in the library.
> >
>
> Or you could add a watch_all flag to make it explicit.
> But fair enough to leave as is - it probably keeps the kernel cleaner.
>
> > > > +/* Possible line status change events */
> > > > +enum {
> > > > + GPIOLINE_CHANGED_REQUESTED = 1,
> > > > + GPIOLINE_CHANGED_RELEASED,
> > > > + GPIOLINE_CHANGED_CONFIG,
> > > > +};
> > > > +
> > > > +/**
> > > > + * struct gpioline_changed - Information about a change in status
> > > > + * of a GPIO line
> > > > + * @line_offset: offset of the line that changed relative to the gpiochip
> > > > + * @event_type: one of GPIOLINE_CHANGED_REQUESTED, GPIOLINE_CHANGED_RELEASED
> > > > + * and GPIOLINE_CHANGED_CONFIG
> > > > + */
> > > > +struct gpioline_changed {
> > > > + __u32 line_offset;
> > > > + __u32 event_type;
> > > > +};
> > > > +
> > >
> > > Rather than sending an event type, and requiring userspace to poll
> > > LINEINFO, which is racy, how about passing the updated info flags here?
> > > A change in the state of the GPIOLINE_FLAG_KERNEL implies the
> > > GPIOLINE_CHANGED_REQUESTED or GPIOLINE_CHANGED_RELEASED, so the
> > > event_type is then redundant.
> > > Userspace would then only have to poll LINEINFO if they were interested
> > > in the consumer on GPIOLINE_CHANGED_REQUESTED.
> > >
> > > To sync kernel and userspace state the current state of each line
> > > should be returned immediately via the fd as soon as the fd is created,
> > > and then subsequently on any change.
> > >
> >
> > I guess you're right. You even made me think we could go as far as to
> > embed the whole gpioline_info structure in struct gpioline_changed.
> > I'd still keep the event type though - otherwise we'd have to assume
> > the user always calls LINEINFO_IOCTL before obtaining the LINECHANGED
> > fd.
> >
>
> I suggested sending the current state of each line as soon as the fd
> is created so that you don't have to assume anything. That initial change
> report is effectively the same as having the user call LINEINFO on each
> line at the same time - without all the race issues.
>
Right, I didn't get this. That means you'd have to call the ioctl()
and immediately follow up with a read(). I'm not a fan of this.
Especially if we were to include a timestamp - what would it refer to?
> I also considered the full gpioline_info structure, as well as reporting
> bulk line changes (replacing line_offset with lineoffsets and num_lines),
> but then I thought it was probably better to keep it minimal and simple.
> The name and consumer are relatively large.
In terms of performance - we're not really degrading it as this is
still not a lot of data.
How about the following:
1. Move the code filling out the struct gpioline_info to a separate
function in the kernel.
2. Call the new ioctl() GET_LINEINFO_FD to indicate it's an extension
of GET_LINEINFO.
3. Embed struct gpioline_info in struct gpioline_info_fd so that the
initial info be read when the file descriptor is created.
4. Likewise embed struct gpioline_info in struct gpioline_changed and
on every status change event provide the whole set of information?
> The name is immutable(??), and so is pointless to include.
It is now, but let's be future-proof. I can imagine having modifiable
line names in the future. The code putting this info in struct
gpioline_info wouldn't be really duplicated and the size of such small
structures doesn't matter much - it's still a single context switch to
read it.
> The consumer only changes when the line is requested, so I was willing
> to live with still having to poll for that.
> And what you gain by reporting bulk lines you might lose in increased
> report size and associated code. OTOH it would make it explicit which
> lines are being changed together...
> So I could well be wrong on that - a full bulk report may be better.
I'm not sure we need bulk reporting - just as we don't provide bulk
GET_GPIOLINE_INFO. The corresponding ioctl() structure would get
pretty complicated soon.
Bartosz
>
> Kent.
>
> > > And a timestamp might be useful, as per gpioevent_data?
> >
> > Sure thing!
> >
> > Bart
> >
> > >
> > > Kent.
> > >
> > > > /* Linerequest flags */
> > > > #define GPIOHANDLE_REQUEST_INPUT (1UL << 0)
> > > > #define GPIOHANDLE_REQUEST_OUTPUT (1UL << 1)
> > > > @@ -176,6 +210,8 @@ struct gpioevent_data {
> > > >
> > > > #define GPIO_GET_CHIPINFO_IOCTL _IOR(0xB4, 0x01, struct gpiochip_info)
> > > > #define GPIO_GET_LINEINFO_IOCTL _IOWR(0xB4, 0x02, struct gpioline_info)
> > > > +#define GPIO_GET_LINECHANGED_FD_IOCTL \
> > > > + _IOWR(0xB4, 0x0b, struct gpioline_changed_fd_request)
> > > > #define GPIO_GET_LINEHANDLE_IOCTL _IOWR(0xB4, 0x03, struct gpiohandle_request)
> > > > #define GPIO_GET_LINEEVENT_IOCTL _IOWR(0xB4, 0x04, struct gpioevent_request)
> > > >
> > > > --
> > > > 2.23.0
> > > >