linux-kernel - Re: Question about TTY_DO_WRITE

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Message-ID: <20081120164719.13daa129@lxorguk.ukuu.org.uk>
Date:	Thu, 20 Nov 2008 16:47:19 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Thomas Pfaff <tpfaff@....com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question about TTY_DO_WRITE_WAKEUP

> > The base code should probably really use test_and_clear_bit() when
> > calling that method.
> 
> Why should you test it, clear it and set it again in n_tty_write_wakeup ?

Because it should only be set again if a wakeup is needed. If the fasync
list for the tty is now empty it should stay clear.

> IMHO a SIGIO on write possible should always be generated if the user wants it, 
> currently it is generated when the user wants it and the tty driver enables the 
> TTY_DO_WRITE_WAKEUP flag. Unfortunately most drivers don't set it.

It is nothing to do with the driver. The line discipline sets it - or
rather should set it. If you have a case where you get an EAGAIN or short
write and the line discipline is not setting it then that is what needs
fixing not the drivers.

> Once a write fails with EAGAIN a flag can be set and only in that case a SIGIO is 
> generated, afterwards the bit is cleared. Maybe that is what TTY_DO_WRITE_WAKEUP 
> was intended for. 

Correct.
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