linux-kernel - Re: [PATCH v2 22/29] nios2: Miscellaneous header files

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Message-ID: <53C50A87.6000902@codesourcery.com>
Date:	Tue, 15 Jul 2014 19:03:35 +0800
From:	Chung-Lin Tang <cltang@...esourcery.com>
To:	Arnd Bergmann <arnd@...db.de>, Ley Foon Tan <lftan@...era.com>
CC:	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <lftan.linux@...il.com>
Subject: Re: [PATCH v2 22/29] nios2: Miscellaneous header files

On 2014/7/15 06:22 PM, Arnd Bergmann wrote:
>> +
>> > +#ifndef _UAPI_ASM_NIOS2_STATFS_H
>> > +#define _UAPI_ASM_NIOS2_STATFS_H
>> > +
>> > +#define __statfs_word __s32
> Why this? Every other architecture except parisc uses the default __u32 here.

Because include/uapi/asm/statfs.h has this:

#ifndef __statfs_word
#if __BITS_PER_LONG == 64
#define __statfs_word __kernel_long_t
#else
#define __statfs_word __u32
#endif
#endif
...
struct statfs64 {
        __statfs_word f_type;
        __statfs_word f_bsize;
        __u64 f_blocks;
        __u64 f_bfree;
...

While the bits/statfs.h header in glibc has this:
struct statfs
  {
    __SWORD_TYPE f_type;
    __SWORD_TYPE f_bsize;
    __field64(__fsblkcnt_t, __fsblkcnt64_t, f_blocks);
...
struct statfs64
  {
    __SWORD_TYPE f_type;
    __SWORD_TYPE f_bsize;
    __fsblkcnt64_t f_blocks;
...

Where __SWORD_TYPE is always a signed integer (int or long int depending
on 32/64-bit word size).  Hence we define the kernel __statfs_word to be
signed 32-bit to match.

Supposedly, tilepro should have a similar issue too, though I guess this
is quite obscure in practice. We so far haven't encountered any issues
on nios2 without this __s32 override either, it's just by observation.

>> > +#include <linux/types.h>
>> > +#include <asm-generic/swab.h>
>> > +
>> > +#ifdef CONFIG_NIOS2_CI_SWAB_SUPPORT
>> > +#ifdef __GNUC__
>> > +
>> > +#define __nios2_swab(x)		\
>> > +	__builtin_custom_ini(CONFIG_NIOS2_CI_SWAB_NO, (x))
>> > +
>> > +static inline __attribute__((const)) __u16 __arch_swab16(__u16 x)
>> > +{
>> > +	return (__u16) __nios2_swab(((__u32) x) << 16);
>> > +}
> Is this actually better than ___constant_swab16()?
> 
> Also, have you checked if you need to support old compiler versions that
> don't have __builtin_bswap16/32/64? With newer compilers you don't need
> to define any of these yourself.
> 
> 	Arnd

This swap is configured/implemented using Nios II's custom instruction
facility; it is not part of the compiler supported Nios II core
instruction set. In fact, even the exact opcode allocated is not fixed.

So the custom swap will not be in a form usable by the compiler, and
hence no use of it during code generation (if it is, you might not even
need to use __builtin_swap*(), there's a optimization pass that
recognizes the shift+or-ing C code patterns)

Thanks,
Chung-Lin

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