linux-kernel - Re: [PATCH 3/4] pps: Use lookup list to reduce ldisc coupling

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Date:	6 Feb 2013 14:34:15 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	giometti@...eenne.com, gregkh@...uxfoundation.org,
	linux@...izon.com, peter@...leysoftware.com
Cc:	jslaby@...e.cz, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH 3/4] pps: Use lookup list to reduce ldisc coupling

> Now that N_TTY uses tty->disc_data for its private data,
> 'subclass' ldiscs cannot use ->disc_data for their own private data.
> 
> Use a lookup list to associate the tty with the pps source.

Thanks for the cleanup.  I fully agree my patch was not a good one;
I just wanted someone more experienced to make the call on rearchitecting.

In particular, I was nervous about getting flamed by Linus for something that
was too ambitious.

One thing I'd prefer to do would be to change:

+static struct pps_device *lookup_pps_by_tty(struct tty_struct *tty,
+					    struct pps_data **p)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&pps_lock, flags);
+	list_for_each_entry((*p), &pps_list, link) {
+		if ((*p)->tty == tty) {
+			spin_unlock_irqrestore(&pps_lock, flags);
+			return (*p)->pps;
+		}
+	}
+	spin_unlock_irqrestore(&pps_lock, flags);
+	return NULL;
+}

to:

static struct pps_data *lookup_pps_by_tty(struct tty_struct *tty)
{
	unsigned long flags;

	spin_lock_irqsave(&pps_lock, flags);
	list_for_each_entry(p, &pps_list, link) {
		if (p->tty == tty)
			break;
	}
	spin_unlock_irqrestore(&pps_lock, flags);
	return p;
}

And do the data->pps dereferencing in the caller.


A more ambitious cleanup would use the existing pps_device list
(maintained to allocate minor device numbers) and add an "owner" field
that can be looked up on, without creating a new data structure and
allocation.

(It could either be a generic "void *", or a "struct device *" and
compare it to tty->dev.)

After all, despite the implementation effort to scale, the total number
of pps devices in a system is usually at most 1 (I have a computer where
I run 2, and I doubt there are many others on the planet who do that.)
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