linux-kernel - Re: + s390-tty-prepare-for-put_char-to-return-success-fail.patch added to -mm tree

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Date:	Tue, 15 Apr 2008 09:21:18 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	alan@...rguk.ukuu.org.uk, alan@...hat.com, borntraeger@...ibm.com,
	peter.oberparleiter@...ibm.com, schwidefsky@...ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: + s390-tty-prepare-for-put_char-to-return-success-fail.patch
	added to -mm tree

>   * NOTE: include/linux/tty_driver.h specifies that a character should be
>   * ignored if there is no room in the queue. This driver implements a different
>   * semantic in that it will block when there is no more room left.
> + *
> + * FIXME: putchar can currently be called from BH and other non blocking
> + * handlers so  this semantic isn't a good idea.
>   */
> -static void
> +static int
>  sclp_vt220_put_char(struct tty_struct *tty, unsigned char ch)
>  {
>  	__sclp_vt220_write(&ch, 1, 0, 0, 1);
> +	return 1;

Uh, oh... the comment above used to mean "block if in schedulable context or
busy wait otherwise". Figuring out which context we were in was done via
in_atomic(), which was broken on !SMP anyway, so that had to go.
So the last parameter for __sclp_vt220_write is now an indicator if scheduling
is allowed or not (1 means yes). Somebody told me that the put_char routine
will only be called from schedulable context, which now doesn't seem to be
true?!
For the console functions we pass indeed 0 to __sclp_vt220_write since printk
may be called within any context. Is that also true for the tty put_char
routine?
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