Re: [PATCH] xen kconfig: clarify INPUT_XEN_KBDDEV_FRONTEND select

From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Jones <drjones@redhat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	linux-arm-kernel@lists.infradead.org,
	Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>,
	Tomi Valkeinen <tomi.valkeinen@ti.com>,
	linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	xen-devel@lists.xensource.com,
	David Vrabel <david.vrabel@citrix.com>,
	boris.ostrovsky@oracle.com
Subject: Re: [PATCH] xen kconfig: clarify INPUT_XEN_KBDDEV_FRONTEND select
Date: Mon, 15 Feb 2016 16:51:08 +0000	[thread overview]
Message-ID: <alpine.DEB.2.02.1602151643150.27008@kaball.uk.xensource.com> (raw)
In-Reply-To: <1455278707-2008263-1-git-send-email-arnd@arndb.de>

CC'ing a few others.

On Fri, 12 Feb 2016, Arnd Bergmann wrote:
> The Xen framebuffer driver selects the xen keyboard driver, so the latter
> will be built-in if XEN_FBDEV_FRONTEND=y. However, when CONFIG_INPUT
> is a loadable module, this configuration cannot work. On mainline kernels,
> the symbol will be enabled but not used, while in combination with
> a patch I have to detect such useless configurations, we get the
> expected link failure:
> 
> drivers/input/built-in.o: In function `xenkbd_remove':
> xen-kbdfront.c:(.text+0x2f0): undefined reference to `input_unregister_device'
> xen-kbdfront.c:(.text+0x30e): undefined reference to `input_unregister_device'
> 
> This changes the dependencies of XEN_FBDEV_FRONTEND so it cannot be
> built-in if CONFIG_INPUT=m && CONFIG_INPUT_MISC=y, as that would result
> in the broken select.
> 
> As usual, we would be much better off without the 'select', but removing
> it now would likely break existing user configurations that depend on
> it, so this adds another hack on top to get it working.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> Fixes: 36c1132e34bd ("xen kconfig: fix select INPUT_XEN_KBDDEV_FRONTEND")
> ---
>  drivers/video/fbdev/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
> index 8ea45a5cd806..fd3d6fd290a9 100644
> --- a/drivers/video/fbdev/Kconfig
> +++ b/drivers/video/fbdev/Kconfig
> @@ -2241,6 +2241,7 @@ config FB_VIRTUAL
>  config XEN_FBDEV_FRONTEND
>  	tristate "Xen virtual frame buffer support"
>  	depends on FB && XEN
> +	depends on INPUT || !INPUT_MISC
>  	select FB_SYS_FILLRECT
>  	select FB_SYS_COPYAREA
>  	select FB_SYS_IMAGEBLIT

This looks very hackish. Couldn't we just do the following?

diff --git a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
index 8ea45a5..3c15f6d 100644
--- a/drivers/video/fbdev/Kconfig
+++ b/drivers/video/fbdev/Kconfig
@@ -2246,7 +2246,7 @@ config XEN_FBDEV_FRONTEND
 	select FB_SYS_IMAGEBLIT
 	select FB_SYS_FOPS
 	select FB_DEFERRED_IO
-	select INPUT_XEN_KBDDEV_FRONTEND if INPUT_MISC
+	select INPUT_XEN_KBDDEV_FRONTEND if (INPUT && INPUT_MISC)
 	select XEN_XENBUS_FRONTEND
 	default y
 	help
