linux-kernel - Re: [PATCH] asm-generic: fix -Wtype-limits compiler warnings

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Date:   Tue, 23 Jul 2019 19:29:36 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, davem@...emloft.net, arnd@...db.de,
        dhowells@...hat.com, jakub@...hat.com, ndesaulniers@...gle.com,
        morbo@...gle.com, jyknight@...gle.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic: fix -Wtype-limits compiler warnings

On Tue, Jul 23, 2019 at 04:49:46PM -0400, Qian Cai wrote:
> The commit d66acc39c7ce ("bitops: Optimise get_order()") introduced a
> compilation warning because "rx_frag_size" is an "ushort" while
> PAGE_SHIFT here is 16. The commit changed the get_order() to be a
> multi-line macro where compilers insist to check all statements in the
> macro even when __builtin_constant_p(rx_frag_size) will return false as
> "rx_frag_size" is a module parameter.
> 
> In file included from ./arch/powerpc/include/asm/page_64.h:107,
>                  from ./arch/powerpc/include/asm/page.h:242,
>                  from ./arch/powerpc/include/asm/mmu.h:132,
>                  from ./arch/powerpc/include/asm/lppaca.h:47,
>                  from ./arch/powerpc/include/asm/paca.h:17,
>                  from ./arch/powerpc/include/asm/current.h:13,
>                  from ./include/linux/thread_info.h:21,
>                  from ./arch/powerpc/include/asm/processor.h:39,
>                  from ./include/linux/prefetch.h:15,
>                  from drivers/net/ethernet/emulex/benet/be_main.c:14:
> drivers/net/ethernet/emulex/benet/be_main.c: In function
> 'be_rx_cqs_create':
> ./include/asm-generic/getorder.h:54:9: warning: comparison is always
> true due to limited range of data type [-Wtype-limits]
>    (((n) < (1UL << PAGE_SHIFT)) ? 0 :  \
>          ^
> drivers/net/ethernet/emulex/benet/be_main.c:3138:33: note: in expansion
> of macro 'get_order'
>   adapter->big_page_size = (1 << get_order(rx_frag_size)) * PAGE_SIZE;
>                                  ^~~~~~~~~
> 
> Fix it by moving almost all of this multi-line macro into a proper
> function __get_order(), and leave get_order() as a single-line macro in
> order to avoid compilation errors.

Wouldn't it just be better to rename __get_order to get_order?

> Fixes: d66acc39c7ce ("bitops: Optimise get_order()")
> Signed-off-by: Qian Cai <cai@....pw>

Other than that, LGTM.

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>