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Date: Thu, 22 Jul 2010 17:20:39 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, linux-fbdev@vger.kernel.org,
        nouveau@lists.freedesktop.org
Subject: Re: [PATCH] vga16fb: refuse to load in face of other driver
 controlling primary card
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On Tue, 20 Jul 2010 21:19:23 +0200
Marcin Slusarz <marcin.slusarz@gmail.com> wrote:

> We don't want vga16fb to mess with hardware initialized by other driver.
> Detect it and refuse to load.
> It fixes nouveau interrupt storm on some machines.
> 
> Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
> ---
>  drivers/video/vga16fb.c |   13 ++++++++++++-
>  1 files changed, 12 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/video/vga16fb.c b/drivers/video/vga16fb.c
> index 28ccab4..4505446 100644
> --- a/drivers/video/vga16fb.c
> +++ b/drivers/video/vga16fb.c
> @@ -22,6 +22,7 @@
>  #include <linux/platform_device.h>
>  #include <linux/screen_info.h>
>  
> +#include <asm/fb.h>
>  #include <asm/io.h>
>  #include <video/vga.h>
>  
> @@ -1415,7 +1416,7 @@ static struct platform_device *vga16fb_device;
>  
>  static int __init vga16fb_init(void)
>  {
> -	int ret;
> +	int ret, i;
>  #ifndef MODULE
>  	char *option = NULL;
>  
> @@ -1424,6 +1425,16 @@ static int __init vga16fb_init(void)
>  
>  	vga16fb_setup(option);
>  #endif
> +	for (i = 0 ; i < FB_MAX; i++) {
> +		if (!registered_fb[i])
> +			continue;
> +		if (fb_is_primary_device(registered_fb[i])) {
> +			printk(KERN_INFO "vga16fb: %s is driving the primary card, refusing to load\n",
> +				registered_fb[i]->fix.id);
> +			return -EBUSY;
> +		}
> +	}
> +
>  	ret = platform_driver_register(&vga16fb_driver);
>  
>  	if (!ret) {

This seems pretty hacky, and specific to vga16fb.

Is vga16fb.c the only fbdev driver (now and forever more) which can
grab the hardware from under a different driver's feet?


This is the basic and very common problem of two drivers diddling the
same hardware, yes?  A common way of preventing such collisions is to
perform a suitably-checked request_region() in both drivers.  Whichever
driver gets there second will get a failure and will bale out cleanly. 
Can that mechanism be made to work in this case?