linux-kernel - Re: [PATCH] bttv: don't compare list

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Date:	Mon, 24 Nov 2008 12:13:14 -0500
From:	"Brian Phelps" <lm317t@...il.com>
To:	"Vegard Nossum" <vegard.nossum@...il.com>
Cc:	"Mauro Carvalho Chehab" <mchehab@...radead.org>,
	"Gerd Knorr" <kraxel@...esex.org>, linux-kernel@...r.kernel.org,
	"Al Viro" <viro@...iv.linux.org.uk>,
	"Mikael Pettersson" <mikpe@...uu.se>,
	"Alexander Shaduri" <ashaduri@...il.com>,
	"Alexey Dobriyan" <adobriyan@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, "Julia Lawall" <julia@...u.dk>
Subject: Re: [PATCH] bttv: don't compare list_head's .next with NULL

This patch seems to cause a select timeout to occur with v4l2's
example capture.c file

On Sat, Nov 22, 2008 at 6:20 AM, Vegard Nossum <vegard.nossum@...il.com> wrote:
> Hi Brian,
>
> Can you see if this patch helps your problem?
>
>
> Vegard
>
>
> From 84396b14b9059de4a697df4ea4e036a22513436e Mon Sep 17 00:00:00 2001
> From: Vegard Nossum <vegard.nossum@...il.com>
> Date: Sat, 22 Nov 2008 12:12:11 +0100
> Subject: [PATCH] bttv: don't compare list_head's .next with NULL
>
> The list implementation doesn't store NULLs in .next/.prev, but
> uses poison values (for-sure invalid pointers). I assume that this
> code wanted to test whether an entry was the last in a list.
>
> This function is only ever called for the video capture list, so
> we know which list to check (it could have been vcapture as well).
>
> Patch is untested!
>
> Signed-off-by: Vegard Nossum <vegard.nossum@...il.com>
> ---
>  drivers/media/video/bt8xx/bttv-risc.c |   10 +++++-----
>  1 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/media/video/bt8xx/bttv-risc.c b/drivers/media/video/bt8xx/bttv-risc.c
> index 5b1b8e4..7a54c99 100644
> --- a/drivers/media/video/bt8xx/bttv-risc.c
> +++ b/drivers/media/video/bt8xx/bttv-risc.c
> @@ -649,14 +649,14 @@ bttv_buffer_activate_video(struct bttv *btv,
>        if (NULL != set->top  &&  NULL != set->bottom) {
>                if (set->top == set->bottom) {
>                        set->top->vb.state    = VIDEOBUF_ACTIVE;
> -                       if (set->top->vb.queue.next)
> +                       if (list_is_last(&set->top->vb.queue, &btv->capture))
>                                list_del(&set->top->vb.queue);
>                } else {
>                        set->top->vb.state    = VIDEOBUF_ACTIVE;
>                        set->bottom->vb.state = VIDEOBUF_ACTIVE;
> -                       if (set->top->vb.queue.next)
> +                       if (list_is_last(&set->top->vb.queue, &btv->capture))
>                                list_del(&set->top->vb.queue);
> -                       if (set->bottom->vb.queue.next)
> +                       if (list_is_last(&set->bottom->vb.queue, &btv->capture))
>                                list_del(&set->bottom->vb.queue);
>                }
>                bttv_apply_geo(btv, &set->top->geo, 1);
> @@ -671,7 +671,7 @@ bttv_buffer_activate_video(struct bttv *btv,
>                      ~0x0f, BT848_COLOR_CTL);
>        } else if (NULL != set->top) {
>                set->top->vb.state  = VIDEOBUF_ACTIVE;
> -               if (set->top->vb.queue.next)
> +               if (list_is_last(&set->top->vb.queue, &btv->capture))
>                        list_del(&set->top->vb.queue);
>                bttv_apply_geo(btv, &set->top->geo,1);
>                bttv_apply_geo(btv, &set->top->geo,0);
> @@ -682,7 +682,7 @@ bttv_buffer_activate_video(struct bttv *btv,
>                btaor(set->top->btswap & 0x0f,   ~0x0f, BT848_COLOR_CTL);
>        } else if (NULL != set->bottom) {
>                set->bottom->vb.state = VIDEOBUF_ACTIVE;
> -               if (set->bottom->vb.queue.next)
> +               if (list_is_last(&set->bottom->vb.queue, &btv->capture))
>                        list_del(&set->bottom->vb.queue);
>                bttv_apply_geo(btv, &set->bottom->geo,1);
>                bttv_apply_geo(btv, &set->bottom->geo,0);
> --
> 1.5.6.5
>
>
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