netdev - Re: [PATCH net-next] ptp: dynamic allocation of PHC char devices

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Message-ID: <1365617559.2581.25.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Wed, 10 Apr 2013 19:12:39 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Miller <davem@...emloft.net>
CC:	<jbenc@...hat.com>, <netdev@...r.kernel.org>,
	<richardcochran@...il.com>
Subject: Re: [PATCH net-next] ptp: dynamic allocation of PHC char devices

On Wed, 2013-04-10 at 13:49 -0400, David Miller wrote:
> From: Ben Hutchings <bhutchings@...arflare.com>
> Date: Wed, 10 Apr 2013 18:37:16 +0100
> 
> > On Wed, 2013-04-10 at 12:47 +0200, Jiri Benc wrote:
> >> As network adapters supporting PTP are becoming more common, machines with
> >> many NICs suddenly have many PHCs, too. The current limit of eight /dev/ptp*
> >> char devices (and thus, 8 network interfaces with PHC) is insufficient. Let
> >> the ptp driver allocate the char devices dynamically.
> >> 
> >> The character devices are allocated in chunks. idr is used for tracking
> >> minor numbers allocation; the mapping from index to pointer is not used, as
> >> it is not needed for posix clocks.
> > [...]
> > 
> > I don't understand why you want to dynamically allocate chunks of minor
> > numbers, rather than the full range.  Unused minors don't seem to cost
> > anything.
> 
> Can't multiple (unrelated) devices carve out minor space in the same
> major?  Isn't that why it's designed this way?

Only at a level above the core char device functions, e.g.
misc_register() for singleton devices.  Unless I'm very much mistaken,
when you allocate a char device range and specify major=0 you get a
dynamically allocated and previously unused major, regardless of how
many minors you asked for.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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