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    SubjectRe: [linux-pm] [PATCH 2/9] PM: suspend_block: Add driver to access suspend blockers from user-space
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    On Sat, Apr 24, 2010 at 7:44 AM, Alan Stern <stern@rowland.harvard.edu> wrote:
    > On Sat, 24 Apr 2010, Pavel Machek wrote:
    >
    >> On Fri 2010-04-23 20:20:47, Arve Hj?nnev?g wrote:
    >> > On Fri, Apr 23, 2010 at 9:43 AM, Alan Stern <stern@rowland.harvard.edu> wrote:
    >> > > On Fri, 23 Apr 2010, Pavel Machek wrote:
    >> > >
    >> > >> Hi!
    >> > >>
    >> > >> > Add a misc device, "suspend_blocker", that allows user-space processes
    >> > >> > to block auto suspend. The device has ioctls to create a suspend_blocker,
    >> > >> > and to block and unblock suspend. To delete the suspend_blocker, close
    >> > >> > the device.
    >> > >> >
    >> > >> > Signed-off-by: Arve Hj??nnev??g <arve@android.com>
    >> > >>
    >> > >> > --- a/Documentation/power/suspend-blockers.txt
    >> > >> > +++ b/Documentation/power/suspend-blockers.txt
    >> > >> > @@ -95,3 +95,20 @@ if (list_empty(&state->pending_work))
    >> > >> >  else
    >> > >> >     suspend_block(&state->suspend_blocker);
    >> > >> >
    >> > >> > +User-space API
    >> > >> > +==============
    >> > >> > +
    >> > >> > +To create a suspend_blocker from user-space, open the suspend_blocker device:
    >> > >> > +    fd = open("/dev/suspend_blocker", O_RDWR | O_CLOEXEC);
    >> > >> > +then call:
    >> > >> > +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_INIT(strlen(name)), name);
    >> > >>
    >> > >>
    >> > >> This seems like very wrong idea -- it uses different ioctl number for
    >> > >> each length AFAICT.
    >> > >
    >> > > How about specifying the name by an ordinary write() call instead of
    >> > > by an ioctl()?
    >> > >
    >> >
    >> > I prefer using ioctls. We have three operations at the moment. Init,
    >> > block and unblock. If we do init with write but block and unblock
    >> > using ioctls, it would be pretty strange. Specifying a command and
    >>
    >> Why would it be "strange"?
    >
    > Why indeed?  Using write() is the natural way to pass a data buffer
    > into the kernel, especially a variable-length buffer.
    >
    > Mixing ioctl() and write() might seem strange at first, but it has
    > plenty of precedent.  Consider adjusting the settings for a serial
    > port, for example.
    >
    That sound like to opposite situation to me. It uses ioctls for setup
    and read/write access the data stream.

    >> > argument in a string to write is more complicated to parse than using
    >> > ioctls.
    >>
    >> More complicated to parse?
    >
    > It shouldn't be -- especially if you assume that the init action must
    > always come first.  The first write would contain the suspend blocker's
    > name; all following writes would have to be either "on" or "off".
    > That's not hard to parse.
    >

    Why should I have to parse a string at all? We already have a control
    interface, ioctl, where user space can pass a command with data. If we
    later want to add other commands we can easily add them without
    breaking existing command. With your interface, where the first write
    is a name, adding more initialization data later becomes harder. I
    also don't like that wring the same string twice has a different
    meaning the second time. With the ioctl interface, you forget to
    initialize the suspend blocker, it block and unblock operations will
    fail. With your interface you create a suspend blocker call "on" or
    "off".

    --
    Arve Hjønnevåg
    --
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