linux-kernel - Re: Sparse warnings on GENMASK + arm32

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Date:   Wed, 26 Jul 2017 09:33:01 -0400 (EDT)
From:   Lance Richardson <lrichard@...hat.com>
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Sparse warnings on GENMASK + arm32

> From: "Stephen Boyd" <sboyd@...eaurora.org>
> To: linux-sparse@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Sent: Tuesday, 25 July, 2017 9:30:20 PM
> Subject: Sparse warnings on GENMASK + arm32
> 
> I see sparse warning when I check a clk driver file in the kernel
> on a 32-bit ARM build.
> 
> drivers/clk/sunxi/clk-sun6i-ar100.c:65:20: warning: cast truncates bits from
> constant value (3ffffffff becomes ffffffff)
> 
> The code in question looks like:
> 
> static const struct factors_data sun6i_ar100_data = {
> 	.mux = 16,
> 	.muxmask = GENMASK(1, 0),
> 	.table = &sun6i_ar100_config,
> 	.getter = sun6i_get_ar100_factors,
> };
> 
> where factors_data is
> 
> struct factors_data {
> 	int enable;
> 	int mux;
> 	int muxmask;
> 	const struct clk_factors_config *table;
> 	void (*getter)(struct factors_request *req);
> 	void (*recalc)(struct factors_request *req);
> 	const char *name;
> };
> 
> 
> and sparse seems to be complaining about the muxmask assignment
> here. Oddly, this doesn't happen on arm64 builds. Both times, I'm
> checking this on an x86-64 machine.
> 
>  $ sparse --version
>  v0.5.1-rc4-1-gfa71b7ac0594
> 
> Is there something confusing to sparse in the GENMASK macro?
> 

Hmm, it seems sparse is incorrectly taking ~0UL to be a 64-bit value
while BITS_PER_LONG is (correctly) evaluated to be 32.

#define GENMASK(h, l) \
	(((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))

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