Re: lsusd - The Linux SUSpend Daemon

[Alan Stern]

On Sun, 23 Oct 2011, NeilBrown wrote:

> > There shouldn't be any trouble about making wakeup_count pollable.  It
> > also would need to respect nonblocking reads, which it currently does 
> > not do.
> 
> Hmm.. you are correct.  I wonder why I thought it did support non-blocking
> reads...
> I guess it was the code for handling an interrupted system call.
> 
> I feel a bit uncomfortable with the idea of sysfs files that block but I
> don't think I can convincingly argue against it.
> A non-blocking flag could be passed in, but it would be a very messy change -
> lots of function call signatures changing needlessly:  we would need a flag
> to the 'show' method ... or add a 'show_nonblock' method which would also be
> ugly.

Right.  Sysfs is pretty inflexible.

> But I think there is a need to block - if there is an in-progress event then
> it must be possible to wait for it to complete as it may not be visible to
> userspace until then.
> We could easily enable 'poll' for wakeup_count and then make it always
> non-blocking, but I'm not really sure I want to require programs to use poll,
> only to allow them.  And without using poll there is no way to wait.
> 
> As wakeup_count really has to be single-access we could possibly fudge
> something by remembering the last value read (like we remember the last value
> written).

A simpler approach would be to add a nonblocking variant:  
/sys/power/wakeup_count_nb.  It would make sense to support poll for 
this file; poll isn't very useful for the wakeup_count file.

Alan Stern

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