On Thu, 9 Dec 2010, Andreas Dilger wrote:
> On 2010-12-09, at 12:20, Greg KH wrote:
> > On Thu, Dec 09, 2010 at 04:25:35PM +0100, Lukas Czerner wrote:
> >> 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.
What's wrong with using libudev ? Well, fist of all I have never heard
about it:), one can argue this is kind of my fault, and second of all
the documentation is kind of non-existent (almost).
But, despite this I did gave libudev a quick try and I must say, it
works, however it is not as simple as calling "ioctl(fd,
BLKGETQUEUEINFO, &val)" as Andreas pointed out.
So, in my use-case, I have a path to the device provided by the user
(strictly speaking it may not be device but for example symbolic link
/dev/mapper/something) and I need to retrieve queue information like
discard_granularity, discard_alignment etc... usually stored in place
like /sys/block/sda/queue/*.
With libudev I need to:
1. create the udev obejct:
udev = udev_new();
if (!udev) {
printf("Can't create udev\n");
exit(1);
}
2. Check the path for the block device
stat(name, &buf);
if (!S_ISBLK(buf.st_mode)) {
printf("Not a block device\n");
exit(1);
}
3. Get udev device object
dev = udev_device_new_from_devnum(udev, 'b', buf.st_rdev);
if (!dev) {
printf("Can not find the device\n");
exit(1);
}
4. Construct path for sysfs attribute I need:
snprintf(path, PATH_MAX, "%s/queue/%s",
udev_device_get_syspath(dev),
"discard_granularity");
5. Open the sysfs file, get page-sized buffer and parse text :-/ (without
checks now):
read(fd, buffer, pagesize);
sscanf(buffer, "%lu", &value);
printf("max_hw_sector_size: %lu\n",value);
Which is opposed to BLKGETQUEUEINFO steps (define val, invoke ioctl,
check result) a bit longer. But I can definitely see you point, it is
feasible and since we have libudev we might want to use this in
userspace. The fact is I would really want to stand up and defend my
ioctl approach, but libudev just might provide what I need without
proceeding the just-another-ioctl-madness on kernel lists :).
Thanks!
-Lukas
> >
> > Ick, no, please just use the sysfs files, don't create a new ioctl, they
> > are horrid.
>
> Can you please show a real example of how using libudev is less horrid than just calling "ioctl(fd, BLKGETQUEUEINFO, &val)"?
>
> How is trying to map a block device name from /etc/mtab (via getmntent()) into a possibly wildly different block device name in /sys (e.g. /dev/vgroot/lvhome vs. /dev/dm-0 vs. /dev/mapper/vgroot-lvhome => /sys/block/dm-0), then parsing text output considered a "good API"?
>
> Cheers, Andreas
>
>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
