red brick + atlantaNix wrote:
On 25 Jan 2006, Matthias Andree prattled cheerily:
Jens Axboe wrote:
Hm. sysfs, procfs, udev, hotplug, netlink (for IPv6) - this all looks rather
complicated and non-portable. I understand that applications that can just
open every device and send SCSI INQUIRY might want to do that on Linux, too.
Applications (already) do this by asking HAL, which can be informed of
new devices in a variety of ways: the up-and-coming one is for the
kernel to notify udevd, following which a udev rule sends a dbus message
to HAL. Everything from the dbus message on up is cross-OS portable.
-scanbus is *totally* unnecessary.
I notice that the first thing people suggest is to make things like
udev, hal and sysfs required instead of optional to do something as
simple as burn a CD. As I mentioned before, if the kernel provided a
list of devices then applications wouldn't break every time a new kernel
came out which needs a new this, and new that, and a few new other
support things.
The kernel could provide a list of devices by category. It doesn't have
to name them, run scripts, give descriptions, or paint them blue. Just a
list of all block devices, tapes, by major/minor and category (ie.
block, optical, floppy) would give the application layer a chance to do
it's own interpretation. HAL is great, but because it's not part of the
kernel it's also going to suffer from "tracking error" for some changes.
I find it easier to teach someone to use -scanbus than to explain how to
make rules for udev.
(Furthermore, it fails to work in a quite laughable fashion in the
presence of hotpluggable storage media. udev handles giving hotpluggable
storage media consistent device names with extreme ease, and tells HAL
about them so that users see the new devices appear even if they don't
have a clue that /dev even exists.
The change that J. Random Nontechnical User will ever run `cdrecord
-scanbus' is *nil*, and applications don't run it either because they
can't judge between all the devices that are listed to pick the one
which is a CD recorder (consider the consequences should they guess
wrong!). Instead, they invariably ask for a device name, or, in more
recent versions get the info from HAL. HAL knows if something is a CD
recorder because its backend, e.g. udev, told it.)
Worth repeating: I find it easier to teach someone to use -scanbus than
to explain how to make rules for udev. HAL is the right answer, but *in*
the kernel, where it will track changes. Since -scanbus tells you a
device is a CDrecorder, or something else, *any user* is likely to be
able to tell it from DCD, CD-ROM, etc. Nice like of text for most devices...
Note: my example of major/minor is just that, almost any presentation
which showed all devices user applications would normally use, in a well
defined way, would address the identifications issue.
--
bill davidsen <[email protected]>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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