linux-kernel - RE: [PATCH 7/7] [v5] drivers/virt: introduce Freescale hypervisormanagement driver

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6D8AD8B@saturn3.aculab.com>
Date:	Fri, 10 Jun 2011 09:32:08 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Arnd Bergmann" <arnd@...db.de>,
	"Timur Tabi" <timur@...escale.com>, <linuxppc-dev@...ts.ozlabs.org>
Cc:	<konrad.wilk@...cle.com>, <kumar.gala@...escale.com>,
	<linux-kernel@...r.kernel.org>, <cmetcalf@...era.com>,
	<akpm@...nel.org>, <linux-console@...r.kernel.org>,
	<greg@...ah.com>, <virtualization@...ts.linux-foundation.org>,
	<alan@...rguk.ukuu.org.uk>
Subject: RE: [PATCH 7/7] [v5] drivers/virt: introduce Freescale hypervisormanagement driver

> > +enum fsl_hv_ioctl_cmd {
> > +       FSL_HV_IOCTL_PARTITION_RESTART = _IOWR(0, 1, struct
fsl_hv_ioctl_restart),
...
> > +};
> 
> Using a #define here is usually preferred because then you 
> can use #ifdef in a user application to check if a given
> value has been assigned.

It is also possible to add:
#define FSL_HV_IOCTL_PARTITION_RESTART FSL_HV_IOCTL_PARTITION_RESTART
to have much the same effect.
But there are many cases where #defines are better.
I only tend to use enums when the constanst are beting generated
by the expansion of a #define.

> More importantly, the code you have chose (0) conflicts with 
> existing drivers (frame buffer, scsi and wavefront among others).
> Please chose a free one and
> add it to Documentation/ioctl/ioctl-number.txt in the same patch.

It is rather a PITA that, when 'int' went from 16 to 32 bits, the
BSD people used the high 16 bits for size/flags rather than
using the extra bits to help make ioctl's unique.
Linux seems to have copied BSD here - rather than SYSV.

One problem with clashing ioctl commands is when systems like
NetBSD are running linux binaries and need to translate ioctl
buffers to/from native format. If the ioctl commands are
unique this can be done much more easily.

	David

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