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SubjectRe: [PATCH 1/7] gnss: add GNSS receiver subsystem
On Tue, Apr 24, 2018 at 06:34:52PM +0200, Johan Hovold wrote:
> +#define GNSS_MINORS 16

Why only 16? Just have to start somewhere?

> +static DEFINE_IDA(gnss_minors);
> +static dev_t gnss_first;
> +
> +/* FIFO size must be a power of two */
> +#define GNSS_READ_FIFO_SIZE 4096
> +#define GNSS_WRITE_BUF_SIZE 1024
> +
> +#define to_gnss_device(d) container_of((d), struct gnss_device, dev)
> +
> +static int gnss_open(struct inode *inode, struct file *file)
> +{
> + struct gnss_device *gdev;
> + int ret = 0;
> +
> + gdev = container_of(inode->i_cdev, struct gnss_device, cdev);
> +
> + get_device(&gdev->dev);
> +
> + nonseekable_open(inode, file);
> + file->private_data = gdev;
> +
> + down_write(&gdev->rwsem);

Just curious, why a rwsem? They can be slower than a normal semaphore,
is this really a contentious lock?

> + if (gdev->disconnected) {
> + ret = -ENODEV;
> + goto unlock;
> + }
> +
> + if (gdev->count++ == 0) {
> + ret = gdev->ops->open(gdev);
> + if (ret)
> + gdev->count--;

Why do we care about the opens here? Do you only want the "child
driver" to have open called once, like a tty driver? Does it matter?

Again, just curious.


> + }
> +unlock:
> + up_write(&gdev->rwsem);
> +
> + if (ret)
> + goto err_put_device;
> +
> + return 0;
> +
> +err_put_device:
> + put_device(&gdev->dev);
> +
> + return ret;

Those last 9 lines can be rewritten as just:

if (!ret)
put_device(&gdev->dev);

return ret;

> +}
> +
> +static int gnss_release(struct inode *inode, struct file *file)
> +{
> + struct gnss_device *gdev = file->private_data;
> +
> + down_write(&gdev->rwsem);
> + if (gdev->disconnected)
> + goto unlock;
> +
> + if (--gdev->count == 0) {
> + gdev->ops->close(gdev);
> + kfifo_reset(&gdev->read_fifo);
> + }
> +unlock:
> + up_write(&gdev->rwsem);
> +
> + put_device(&gdev->dev);
> +
> + return 0;
> +}
> +
> +static ssize_t gnss_read(struct file *file, char __user *buf,
> + size_t count, loff_t *pos)
> +{
> + struct gnss_device *gdev = file->private_data;
> + unsigned int copied;
> + int ret;
> +
> + mutex_lock(&gdev->read_mutex);
> + while (kfifo_is_empty(&gdev->read_fifo)) {
> + mutex_unlock(&gdev->read_mutex);
> +
> + if (gdev->disconnected)
> + return 0;
> +
> + if (file->f_flags & O_NONBLOCK)
> + return -EAGAIN;
> +
> + ret = wait_event_interruptible(gdev->read_queue,
> + gdev->disconnected ||
> + !kfifo_is_empty(&gdev->read_fifo));
> + if (ret)
> + return -ERESTARTSYS;
> +
> + mutex_lock(&gdev->read_mutex);
> + }
> +
> + ret = kfifo_to_user(&gdev->read_fifo, buf, count, &copied);
> + if (ret)
> + goto out_unlock;
> +
> + ret = copied;
> +
> + dev_dbg(&gdev->dev, "%s - count = %zu\n", __func__, copied);
> +
> +out_unlock:
> + mutex_unlock(&gdev->read_mutex);
> +
> + return ret;
> +}
> +
> +static ssize_t gnss_write(struct file *file, const char __user *buf,
> + size_t count, loff_t *pos)
> +{
> + struct gnss_device *gdev = file->private_data;
> + size_t written = 0;
> + int ret;
> +
> + dev_dbg(&gdev->dev, "%s - count = %zu\n", __func__, count);

Nit, some of these initial dev_dbg() calls might be able to be removed,
as ftrace should handle the tracing code now, right?

> +
> + if (gdev->disconnected) {
> + dev_dbg(&gdev->dev, "%s - disconnected\n", __func__);
> + return -EIO;
> + }
> +
> + if (!count)
> + return 0;
> +
> + if (!gdev->ops->write_raw)
> + return -EIO;
> +
> + /* Ignoring O_NONBLOCK, write_raw() is synchronous. */
> +
> + ret = mutex_lock_interruptible(&gdev->write_mutex);
> + if (ret)
> + return -ERESTARTSYS;
> +
> + for (;;) {
> + size_t n = count - written;
> +
> + if (n > GNSS_WRITE_BUF_SIZE)
> + n = GNSS_WRITE_BUF_SIZE;
> +
> + if (copy_from_user(gdev->write_buf, buf, n)) {
> + ret = -EFAULT;
> + goto out_unlock;
> + }
> +
> + /*
> + * Assumes write_raw can always accept GNSS_WRITE_BUF_SIZE
> + * bytes.
> + *
> + * FIXME: revisit
> + */
> + down_read(&gdev->rwsem);
> + if (!gdev->disconnected)
> + ret = gdev->ops->write_raw(gdev, gdev->write_buf, n);
> + else
> + ret = -EIO;
> + up_read(&gdev->rwsem);
> +
> + if (ret < 0)
> + break;
> +
> + written += ret;
> + buf += ret;
> +
> + if (written == count)
> + break;
> + }
> +
> + if (written)
> + ret = written;
> +out_unlock:
> + mutex_unlock(&gdev->write_mutex);
> +
> + return ret;
> +}
> +
> +static __poll_t gnss_poll(struct file *file, poll_table *wait)
> +{
> + struct gnss_device *gdev = file->private_data;
> + __poll_t mask = 0;
> +
> + poll_wait(file, &gdev->read_queue, wait);
> +
> + if (!kfifo_is_empty(&gdev->read_fifo))
> + mask |= EPOLLIN | EPOLLRDNORM;
> + if (gdev->disconnected)
> + mask |= EPOLLHUP;
> +
> + dev_dbg(&gdev->dev, "%s - mask = 0x%04x\n", __func__, mask);
> +
> + return mask;
> +}
> +
> +static const struct file_operations gnss_fops = {
> + .owner = THIS_MODULE,
> + .open = gnss_open,
> + .release = gnss_release,
> + .read = gnss_read,
> + .write = gnss_write,
> + .poll = gnss_poll,
> + .llseek = no_llseek,
> +};
> +
> +static struct class *gnss_class;
> +
> +static void gnss_device_release(struct device *dev)
> +{
> + struct gnss_device *gdev = to_gnss_device(dev);
> +
> + dev_dbg(dev, "%s\n", __func__);
> +
> + kfree(gdev->write_buf);
> + kfifo_free(&gdev->read_fifo);
> + ida_simple_remove(&gnss_minors, gdev->id);
> + kfree(gdev);
> +}
> +
> +struct gnss_device *gnss_allocate_device(struct device *parent)
> +{
> + struct gnss_device *gdev;
> + struct device *dev;
> + int id;
> + int ret;
> +
> + gdev = kzalloc(sizeof(*gdev), GFP_KERNEL);
> + if (!gdev)
> + return NULL;
> +
> + id = ida_simple_get(&gnss_minors, 0, GNSS_MINORS, GFP_KERNEL);
> + if (id < 0) {
> + kfree(gdev);
> + return ERR_PTR(id);
> + }
> +
> + gdev->id = id;
> +
> + dev = &gdev->dev;
> + device_initialize(dev);
> + dev->devt = gnss_first + id;
> + dev->class = gnss_class;
> + dev->parent = parent;
> + dev->release = gnss_device_release;
> + dev_set_drvdata(dev, gdev);
> + dev_set_name(dev, "gnss%d", id);
> +
> + init_rwsem(&gdev->rwsem);
> + mutex_init(&gdev->read_mutex);
> + mutex_init(&gdev->write_mutex);
> + init_waitqueue_head(&gdev->read_queue);
> +
> + ret = kfifo_alloc(&gdev->read_fifo, GNSS_READ_FIFO_SIZE, GFP_KERNEL);
> + if (ret)
> + goto err_put_device;
> +
> + gdev->write_buf = kzalloc(GNSS_WRITE_BUF_SIZE, GFP_KERNEL);
> + if (!gdev->write_buf)
> + goto err_put_device;
> +
> + cdev_init(&gdev->cdev, &gnss_fops);
> + gdev->cdev.owner = THIS_MODULE;

This protects this module from being unloaded, but how do you pass on
the module reference counts to the "child" gnss driver? Am I just
missing that logic here somewhere?

Anyway, minor things, this looks really clean, nice job.

greg k-h

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