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    SubjectRe: SYSFS: need a noncaching read
    On Wednesday September 12, nickpiggin@yahoo.com.au wrote:
    > On Wednesday 12 September 2007 20:01, Greg KH wrote:
    > > On Wed, Sep 12, 2007 at 07:32:07AM +0200, Robert Schwebel wrote:
    > > > On Tue, Sep 11, 2007 at 11:43:17AM +0200, Heiko Schocher wrote:
    > > > > I have developed a device driver and use the sysFS to export some
    > > > > registers to userspace.
    > > >
    > > > Uuuh, uggly. Don't do that. Device drivers are there to abstract things,
    > > > not to play around with registers from userspace.
    > > >
    > > > > I opened the sysFS File for one register and did some reads from this
    > > > > File, but I alwas becoming the same value from the register, whats not
    > > > > OK, because they are changing. So I found out that the sysFS caches
    > > > > the reads ... :-(
    > > >
    > > > Yes, it does. What you can do is close()ing the file handle between
    > > > accesses, which makes it work but is slow.
    > >
    > > Do an lseek back to 0 and then re-read, you will get called in your
    > > driver again.
    >
    > Can you do a pread with offset 0 to avoid the two syscalls? (which some
    > people seem to be concerned about)

    No.
    Looking in fs/sysfs/file.c, we notice the field "needs_read_fill" in
    struct sysfs_buffer.

    sysfs_read_file will only call fill_read_buffer (which calls the
    ->show routine) if this is 1;

    It is cleared by fill_read_buffer and set to 1:
    - at open
    - by fill_write_buffer (i.e. if you write to the file descriptor)
    - by sysfs_poll when an event was detected.

    So currently you cannot simply open a sysfs file an read multiple
    times.

    One option would be to call fill_read_buffer if *ppos == 0.
    I cannot see that being a problem in practice, but maybe there is a
    reason why it wasn't done that way.
    Another option might be to call fill_read_buffer also if
    buffer->event != atomic_read(&attr_sd->s_event)
    and require drivers to call sysfs_notify when they make a change that
    should be noticed. But I doubt that is really important.

    NeilBrown
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