linux-kernel - Re: [PATCH] fix BUILD_BUG_ON() and a couple of bogus uses of it

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Date:	Thu, 17 Sep 2009 17:01:56 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	akpm@...ux-foundation.org
Cc:	JBeulich@...ell.com, linux-kernel@...r.kernel.org,
	sparclinux@...r.kernel.org
Subject: Re: [PATCH] fix BUILD_BUG_ON() and a couple of bogus uses of it

From: Andrew Morton <akpm@...ux-foundation.org>
Date: Thu, 17 Sep 2009 16:24:37 -0700

> sparc64:
> 
> In file included from arch/sparc/kernel/vio.c:17:
> /usr/src/devel/arch/sparc/include/asm/vio.h: In function `vio_dring_avail':
> /usr/src/devel/arch/sparc/include/asm/vio.h:261: error: bit-field `<anonymous>' width not an integer constant
> 
> static inline u32 vio_dring_avail(struct vio_dring_state *dr,
> 				  unsigned int ring_size)
> {
> 	BUILD_BUG_ON(!is_power_of_2(ring_size));
> 
> 	return (dr->pending -
> 		((dr->prod - dr->cons) & (ring_size - 1)));
> }
> 
> changing it to MAYBE_BUILD_BUG_ON seems to have fixed it.

That's completely bogus.

First of all, arch/sparc/kernel/vio.c never calls this function
so it should never be evaluated.

Second of all, all the places that do call this function only
pass pure constants as the 'ring_size' parameter.

drivers/block/sunvdc.c:

static inline u32 vdc_tx_dring_avail(struct vio_dring_state *dr)
{
	return vio_dring_avail(dr, VDC_TX_RING_SIZE);
}

drivers/net/sunvnet.c:

static inline u32 vnet_tx_dring_avail(struct vio_dring_state *dr)
{
	return vio_dring_avail(dr, VNET_TX_RING_SIZE);
}

Making this MAYBE_BUILD_BUG_ON shouldn't be necessary.
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