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    SubjectRe: [RFC PATCH 0/2 v1] Ioctl for reading block queue information
    On Thu, Dec 09, 2010 at 04:25:35PM +0100, Lukas Czerner wrote:
    > Hi all,
    >
    > For a long time it has been pretty painful to retrieve informations from
    > /sys/block/*/queue for particular block device. Not only it is painful
    > to retrieve informations within C tool, parsing strings, etc, but one
    > have to run into problem even finding the proper path in sysfs.

    What's wrong with using libudev? That should give you all of this
    information easily using a .c program without any need to change the
    kernel at all.

    > This patch set adds new block ioctl BLKGETQUEUEINFO, which is designed to
    > retrieve particular block queue attributes. I have tried to not to
    > create completely new interface, but since we already have those
    > information within block queue sysfs kobjects, rather retrieve it from
    > there.

    Ick, no, please just use the sysfs files, don't create a new ioctl, they
    are horrid.


    >
    > So it binds enum blkq_info_type in include/linux/fs.h with
    > default_attrs[] definition within block/blk-sysfs.c for ordering
    > purposes, so the userspace has reliable way to address specific
    > block queue attribute.
    >
    > In order to avoid string parsing (given that 99% of block queue sysfs
    > attributes are of unsigned long type) I have introduced new sysfs_ops
    > member *get which may be used to pass pointer to any type of data (unsigned
    > long in this case). Then this get method is used to withdraw information
    > through kobject->ktype->sysfs_ops.
    >
    > Although there are some downsides of this approach I would like to discuss:
    >
    > 1. In current state BLKGETQUEUEINFO and the whole infrastructure allow to
    > retrieve only numeric values (no text), this means we are no able to
    > retrieve schedule information.
    >
    > 2. Adding new *get member into sysfs_ops. I see this as the most painful
    > downside, however, when we have more subsystems adding ioctls like this,
    > they can easily use this method, the same way I am using it now.

    Nope, don't do this, just use sysfs how it is supposed to be used, from
    userspace.

    sorry,

    greg k-h


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