linux-kernel - Re: [Patch 14/16] Remove needless kmalloc casts in the zd1211rw drivers.

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Message-ID: <46AFEB3C.6020601@gentoo.org>
Date:	Tue, 31 Jul 2007 22:09:00 -0400
From:	Daniel Drake <dsd@...too.org>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
CC:	jack@...keye.stone.uk.eu.org, linux-kernel@...r.kernel.org,
	trivial@...nel.org
Subject: Re: [Patch 14/16] Remove needless kmalloc casts in the zd1211rw drivers.

Stefan Richter wrote:
>> NAK: this patch adds a sparse warning
>> zd_chip.c:116:15: warning: implicit cast to nocast type
> 
> What does this warning mean?  a16 is defined as zd_addr_t *.

zd_addr_t is defined as
typedef u16 __nocast zd_addr_t;

The __nocast annotation forces us to use explicit casts for values of 
this type. This is really useful as we have functions which take an 
address and a value (e.g. register writes). It's very easy to get them 
in the wrong order and have a non-obvious bug. With this mechanism, 
sparse helps us avoid this.

The case with pointers may be a little unintuitive, but it's a small 
price to pay for a bug that bit us hard once before...

The number of other places we have to cast is very small due to use of 
the preprocessor. For example we have:

#define ZD_ADDR(base, offset) \
	((zd_addr_t)(((base) & ADDR_BASE_MASK) | ((offset) & ADDR_OFFSET_MASK)))

#define CTL_REG(offset)  ZD_ADDR(CR_BASE, offset)	/* byte addressing */

Then we have a few hundred registers defined in terms of CTL_REG.

> Also, why is there this cast right before that?
> 
> 	zd_addr_t *a16 = (zd_addr_t *)NULL;

For similar reasons.

Daniel
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