linux-kernel - Re: [linux-pm] [PATCH 2/9] PM: suspend_block: Add driver to access suspend blockers from user-space

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Message-ID: <v2ld6200be21004251534y6c820e96l824f3288201a757d@mail.gmail.com>
Date:	Sun, 25 Apr 2010 15:34:20 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Magnus Damm <damm@...l.co.jp>,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [linux-pm] [PATCH 2/9] PM: suspend_block: Add driver to access 
	suspend blockers from user-space

On Sat, Apr 24, 2010 at 7:44 AM, Alan Stern <stern@...land.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@...land.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@...roid.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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