Date:	Tue, 25 Mar 2008 15:44:00 +0100
From:	Rodolfo Giometti <giometti@...eenne.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, dwmw2@...radead.org,
	davej@...hat.com, sam@...nborg.org, greg@...ah.com,
	randy.dunlap@...cle.com
Subject: Re: [PATCH 1/7] LinuxPPS core support.

On Thu, Mar 20, 2008 at 01:03:56PM -0700, Andrew Morton wrote:
> On Thu,  6 Mar 2008 13:09:00 +0100
> Rodolfo Giometti <giometti@...ux.it> wrote:
> 
> > +pps_core-y			:= pps.o kapi.o sysfs.o
> 
> Does it compile OK with CONFIG_SYSFS=n?

Yes. Tested.

> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/init.h>
> > +#include <linux/sched.h>
> > +#include <linux/time.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/idr.h>
> > +#include <linux/fs.h>
> > +#include <linux/pps.h>
> > +
> > +/*
> > + * Global variables
> > + */
> > +
> > +DEFINE_SPINLOCK(idr_lock);
> 
> This name is insufficiently specific.  Not only does it risk linkage
> errors, it makes it ahrd for poeple to work out where the symbol came from.
> 
> I renamed it to pps_idr_lock.

Fixed.

> > +DEFINE_IDR(pps_idr);
> >
> > ...
> >
> > +void pps_unregister_source(int source)
> > +{
> > +	struct pps_device *pps;
> > +
> > +	spin_lock_irq(&idr_lock);
> > +	pps = idr_find(&pps_idr, source);
> > +
> > +	if (!pps) {
> > +		spin_unlock_irq(&idr_lock);
> > +		return;
> > +	}
> > +
> > +	/* This should be done first in order to deny IRQ handlers
> > +	 * to access PPS structs
> > +	 */
> > +
> > +	idr_remove(&pps_idr, pps->id);
> > +	spin_unlock_irq(&idr_lock);
> > +
> > +	wait_event(pps->usage_queue, atomic_read(&pps->usage) == 0);
> > +
> > +	pps_sysfs_remove_source_entry(pps);
> > +	pps_unregister_cdev(pps);
> > +	kfree(pps);
> > +}
> > +EXPORT_SYMBOL(pps_unregister_source);
> 
> The wait_event() stuff really shouldn't be here: it should be integral to
> the refcounting:
> 
> void pps_dev_put(struct pps_device *pps)
> {
> 	spin_lock_irq(&pps_lock);
> 	if (atomic_dec_and_test(&pps->usage))
> 		idr_remove(&pps_idr, pps->id);
> 	else
> 		pps = NULL;
> 	spin_unlock_irq(&pps_lock);
> 	if (pps) {
> 		/*
> 		 * Might need to do the below via schedule_work() if
> 		 * pps_dev_put() is to be callable from atomic context
> 		 */
> 		pps_sysfs_remove_source_entry(pps);
> 		pps_unregister_cdev(pps);
> 		kfree(pps);
> 	}
> }

I don't understand where I should use this function... :'(

> 
> As it stands, there might be deadlocks such as when a process which itself
> holds a ref on the pps_device (with an open fd?) calls
> pps_unregister_source.

I can add a wait_event_interruptible in order to allow userland to
continue by receiving a signal. It could be acceptable?

> Also, we need to take care that all processes which were waiting in
> pps_unregister_source() get to finish their cleanup before we permit rmmod
> to proceed.  Is that handled somewhere?

I don't understand the problem... this code as been added in order to
avoid the case where a pps_event() is called while a process executes
the pps_unregister_source(). If more processes try to execute this
code the first which enters will execute idr_remove() which prevents
another process to reach the wait_event()... is that wrong? =:-o

> 
> > +void pps_event(int source, int event, void *data)
> 
> Please document the API in the kernel source.  I realise there's a teeny
> bit of documentation in pps.txt, but people don't think to look there and
> it tends to go out of date.
> 
> It doesn't have to be fancy formal kerneldoc - it's better to add *good*
> comments which tell people what they need to know.  For some reason people
> seem to add useless obvious stuff when they do their comments in kerneldoc
> form.

Here my proposal. It could be enought? :)

diff --git a/drivers/pps/kapi.c b/drivers/pps/kapi.c
index 6cac7b8..34b3b22 100644
--- a/drivers/pps/kapi.c
+++ b/drivers/pps/kapi.c
@@ -59,6 +59,18 @@ static void pps_add_offset(struct pps_ktime *ts, struct pps_ktime *offset)
  * Exported functions
  */
 
+/* pps_register_source - add a PPS source in the system
+ *
+ *	info: the PPS info struct
+ *	default_params: the default PPS parameters of the new source
+ *
+ * This function is used to add a new PPS source in the system. The new
+ * source is described by info's fields and it will have, as default PPS
+ * parameters, the ones specified into default_params.
+ *
+ * The function returns, in case of success, the PPS source ID.
+ */
+
 int pps_register_source(struct pps_source_info *info, int default_params)
 {
 	struct pps_device *pps;
@@ -151,6 +163,14 @@ pps_register_source_exit:
 }
 EXPORT_SYMBOL(pps_register_source);
 
+/* pps_unregister_source - remove a PPS source from the system
+ *
+ *	source: the PPS source ID
+ *
+ * This function is used to remove a previously registered PPS source from
+ * the system.
+ */
+
 void pps_unregister_source(int source)
 {
 	struct pps_device *pps;
@@ -177,6 +197,20 @@ void pps_unregister_source(int source)
 }
 EXPORT_SYMBOL(pps_unregister_source);
 
+/* pps_event - register a PPS event into the system
+ *
+ *	source: the PPS source ID
+ *	event: the event type
+ *	data: userdef pointer
+ *
+ * This function is used by each PPS client in order to register a new
+ * PPS event into the system (it's usually called inside an IRQ handler).
+ *
+ * If an echo function is associated with the PPS source it will be called
+ * as:
+ *	pps->info.echo(source, event, data);
+ */
+
 void pps_event(int source, int event, void *data)
 {
 	struct pps_device *pps;

> > +{
> > +	struct pps_device *pps;
> > +	struct timespec __ts;
> > +	struct pps_ktime ts;
> > +	unsigned long flags;
> > +
> > +	/* First of all we get the time stamp... */
> > +	getnstimeofday(&__ts);
> > +
> > +	/* ... and translate it to PPS time data struct */
> > +	ts.sec = __ts.tv_sec;
> > +	ts.nsec = __ts.tv_nsec;
> > +
> > +	if ((event & (PPS_CAPTUREASSERT | PPS_CAPTURECLEAR)) == 0) {
> > +		printk(KERN_ERR "pps: unknown event (%x) for source %d\n",
> > +			event, source);
> > +		return;
> > +	}
> > +
> > +	spin_lock_irqsave(&idr_lock, flags);
> > +	pps = idr_find(&pps_idr, source);
> > +
> > +	/* If we find a valid PPS source we lock it before leaving
> > +	 * the lock!
> > +	 */
> > +	if (pps)
> > +		atomic_inc(&pps->usage);
> > +	spin_unlock_irqrestore(&idr_lock, flags);
> 
> The above pattern is repeated rather a lot and could perhaps be extracted
> into a nice pps_dev_get() helper.

It could be... but, is it mandatory? ;)

> > +	if (!pps)
> > +		return;
> > +
> > +	pr_debug("PPS event on source %d at %llu.%06u\n",
> > +			pps->id, ts.sec, ts.nsec);
> > +
> > +	spin_lock_irqsave(&pps->lock, flags);
> > +
> > +	/* Must call the echo function? */
> > +	if ((pps->params.mode & (PPS_ECHOASSERT | PPS_ECHOCLEAR)))
> > +		pps->info.echo(source, event, data);
> > +
> > +	/* Check the event */
> > +	pps->current_mode = pps->params.mode;
> > +	if (event & PPS_CAPTUREASSERT) {
> > +		/* We have to add an offset? */
> > +		if (pps->params.mode & PPS_OFFSETASSERT)
> > +			pps_add_offset(&ts, &pps->params.assert_off_tu);
> > +
> > +		/* Save the time stamp */
> > +		pps->assert_tu = ts;
> > +		pps->assert_sequence++;
> > +		pr_debug("capture assert seq #%u for source %d\n",
> > +			pps->assert_sequence, source);
> > +	}
> > +	if (event & PPS_CAPTURECLEAR) {
> > +		/* We have to add an offset? */
> > +		if (pps->params.mode & PPS_OFFSETCLEAR)
> > +			pps_add_offset(&ts, &pps->params.clear_off_tu);
> > +
> > +		/* Save the time stamp */
> > +		pps->clear_tu = ts;
> > +		pps->clear_sequence++;
> > +		pr_debug("capture clear seq #%u for source %d\n",
> > +			pps->clear_sequence, source);
> > +	}
> > +
> > +	pps->go = ~0;
> > +	wake_up_interruptible(&pps->queue);
> > +
> > +	kill_fasync(&pps->async_queue, SIGIO, POLL_IN);
> > +
> > +	spin_unlock_irqrestore(&pps->lock, flags);
> > +
> > +	/* Now we can release the PPS source for (possible) deregistration */
> > +	spin_lock_irqsave(&idr_lock, flags);
> > +	atomic_dec(&pps->usage);
> > +	wake_up_all(&pps->usage_queue);
> > +	spin_unlock_irqrestore(&idr_lock, flags);
> > +}
> > +EXPORT_SYMBOL(pps_event);
> >
> > ...
> >
> > +static int pps_cdev_open(struct inode *inode, struct file *file)
> > +{
> > +	struct pps_device *pps = container_of(inode->i_cdev,
> > +						struct pps_device, cdev);
> > +	int found;
> > +
> > +	spin_lock_irq(&idr_lock);
> > +	found = idr_find(&pps_idr, pps->id) != NULL;
> > +
> > +	/* Lock the PPS source against (possible) deregistration */
> > +	if (found)
> > +		atomic_inc(&pps->usage);
> > +
> > +	spin_unlock_irq(&idr_lock);
> 
> That looks a bit odd.  How can the pps_device not be registered in the IDR
> tree at this stage?

This is needed in this scenario: a process just enters into
pps_unregister_source() while another one is executing pps_cdev_open()
on the same PPS source. We cannot open an unregistered source...

> 
> 
> > +	if (!found)
> > +		return -ENODEV;
> > +
> > +	file->private_data = pps;
> > +
> > +	return 0;
> > +}
> >
> > ...
> >
> > +/* Kernel consumers */
> > +#define PPS_KC_HARDPPS		0	/* hardpps() (or equivalent) */
> > +#define PPS_KC_HARDPPS_PLL	1	/* hardpps() constrained to
> > +					   use a phase-locked loop */
> > +#define PPS_KC_HARDPPS_FLL	2	/* hardpps() constrained to
> > +					   use a frequency-locked loop */
> > +/*
> > + * Here begins the implementation-specific part!
> > + */
> > +
> > +struct pps_fdata {
> > +	struct pps_kinfo info;
> > +	struct pps_ktime timeout;
> > +};
> > +
> > +#include <linux/ioctl.h>
> > +
> > +#define PPS_CHECK		_IO('P', 0)
> > +#define PPS_GETPARAMS		_IOR('P', 1, struct pps_kparams *)
> > +#define PPS_SETPARAMS		_IOW('P', 2, struct pps_kparams *)
> > +#define PPS_GETCAP		_IOR('P', 3, int *)
> > +#define PPS_FETCH		_IOWR('P', 4, struct pps_fdata *)
> > +
> > +#ifdef __KERNEL__
> > +
> > +#include <linux/cdev.h>
> > +#include <linux/device.h>
> > +
> > +#define PPS_VERSION		"5.0.0"
> > +#define PPS_MAX_SOURCES		16		/* should be enough... */
> 
> It's nice to avoid sprinkling #includes throughout the file, please. 
> People expect to be able to see what's being included in the first
> screenful of the file.

Fixed.

> > +/*
> > + * Global defines
> > + */
> > +
> > +/* The specific PPS source info */
> > +struct pps_source_info {
> > +	char name[PPS_MAX_NAME_LEN];		/* simbolic name */
> > +	char path[PPS_MAX_NAME_LEN];		/* path of connected device */
> > +	int mode;				/* PPS's allowed mode */
> > +
> > +	void (*echo)(int source, int event, void *data); /* PPS echo function */
> > +
> > +	struct module *owner;
> > +	struct device *dev;
> > +};
> > +
> 

I just fixed all your suggestions (apart what reported above) and I'm
ready to propose a new patch set.

Please let me know what else should I fix and if the proposed inline
documentation is ok.

Ciao,

Rodolfo

-- 

GNU/Linux Solutions                  e-mail:    giometti@...eenne.com
Linux Device Driver                             giometti@...dd.com
Embedded Systems                     		giometti@...ux.it
UNIX programming                     phone:     +39 349 2432127
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