[dm-devel] Re: no INQUIRY from userspace please

[Stefan Richter]

(adding Cc linux-hotplug)

James Bottomley wrote:
> On Thu, 2008-02-07 at 11:08 +0100, Stefan Richter wrote:
>> Mike Anderson wrote:
>> > A number of user apps like lvm scanning that execute media access commands
>> > already have filter capability to filter devices that one does not want to
>> > scan. Another class of device scanners just use inquiries which are not
>> > effected by the passive state (though some could probably use udevinfo and
>> > reduce the amount of repeated SCSI inquiries execute on the system.
>> 
>> To expand on this:
>> 
>> At least on desktop systems and SOHO server systems, userspace should
>> _never_ issue INQUIRY.  There are too many broken firmwares out there
>> which assume that there will never be more than one INQUIRY sent.  They
>> start to return garbled data or crash if they get a second INQUIRY.
> 
> It's all very well to say this, but I think if you look at what udev
> does, you'll find that it uses scsi_id to send a VPD inquiry to the
> device so it can populate /dev/disk/by-id, so the point is already
> conceded (and I think looking at a recent camera crash that seems to
> have been precipitated by this, it's already causing us problems).
> 
> This is all a tradeoff.  If you want userspace *never* to issue raw SCSI
> commands like INQUIRY, we're going to have to provide the needed
> information from the kernel via sysfs ... including VPD strings.  This
> is something we've always shovelled off into userspace before.

Well, it's definitely awkward to have to deal with less than perfect
firmwares.

But there is still potential to optimize udev based on what we already
have in sysfs.  For example, on Gentoo I see calls of scsi_id and usb_id
for FireWire disks (by udev? by HAL? I don't know), even though udev
actually also reads the FireWire driver specific sysfs attribute with
the target port ID and LUN.  I.e. udev knows where to look but calls
pointless helpers anyway.

(BTW, how about sysfs attributes for the target port identifier and for
the logical unit identifier --- at transport independent sysfs paths ---
to simplify userspace's life?  What else?)
-- 
Stefan Richter
-=====-==--- --=- --===
http://arcgraph.de/sr/