ioctls, etc. (was Re: [PATCH 1/4] sas: add flag for locally attached PHYs)

Jeff Garzik
2005-10-21T00:46:40Z

Andrew Patterson wrote:
> I believe there is a common understanding that IOCTL's are bad and
> should be avoided. See:
>
> http://lkml.org/lkml/2001/5/20/81

Yep. Linus's rant here reflects not only his opinion, but general
consensus, I feel.

> Yes, CSMI should have had more Linux input when it was developed. The
> no-new IOCTL policy certainly came as a surprise to the authors. Still,
> there doesn't seem to be any other usable cross-platform interface that
> is acceptable to the linux community (or at least to Christoph)'s corner

Beyond Linus's rant, ioctls have a practical limitation, too: because
they are untyped, we have to deal with stuff like the 32<->64 compat
ioctl thunking.

Consider what an ioctl is, overall: a domain-specific "do this
operation" interface. Which, further, is nothing but a wrapping of a
"send message" + "receive response" interface. There are several ways
to do this in Linux:

* block driver. a block driver is nothing but a message queue. This is
why James has suggested implementing SMP as a block driver. People get
stuck into thinking "block driver == block device", which is wrong. The
Linux block layer is nothing but a message queueing interface.

* netlink. You connect to <an object> and send/receive messages. Not
strictly limited to networking, as this is used in some areas of the
kernel now for generic event delivery.

* char driver. Poor man's netlink. Unless its done right, it suffers
from the same binary problems as ioctls. I don't recommend this path.

* sysfs. This has no inherent message/queue properties by itself, and
is less structured than blockdrvr or netlink, so dealing with more than
one outstanding operation at a time requires some coding.

sysfs's attractiveness is in its ease of use. It works with standard
Unix filesystem tools. You don't need to use a library to read
information, cat(1) often suffices. sysfs, since its normal ASCII
(UTF8), also has none of the annoying 32<->64 compatibility issues that
ioctls suffer from.

Which is best? I don't have a good answer. Largely depends on the
situation, particularly queueing needs. Networking and storage are
rapidly converging into "messaging", so the situation is highly fluid in
any case. Coming from a networking background, I sorta lean towards the
solution noone has attempted yet: netlink.

> of it). My personal preference is to hide this stuff in a library, so
> the kernel implementation is hidden. But even a library needs an
> underlying kernel implementation.

Strongly agree here. libc shelters userspace from [most] kernel
changes, by exporting syscalls in a standard manner. alsa-lib was
created to shelter userspace from current and future changes in the
kernel audio interface. libsdi is quite reasonable.

Jeff