linux-kernel - Re: [PATCH v1] x86/io: Define readq()/writeq() to use 64-bit type

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Date:   Mon, 22 Jan 2018 16:46:59 -0800
From:   hpa@...or.com
To:     "Mehta, Sohil" <sohil.mehta@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "mitake@....info.waseda.ac.jp" <mitake@....info.waseda.ac.jp>
Subject: Re: [PATCH v1] x86/io: Define readq()/writeq() to use 64-bit type

On January 22, 2018 4:32:14 PM PST, "Mehta, Sohil" <sohil.mehta@...el.com> wrote:
>On Fri, 2018-01-19 at 16:33 +0200, Andy Shevchenko wrote:
>> Since non atomic readq() and writeq() were added some of the drivers
>> would like to use it in a manner of:
>> 
>>  #include <io-64-nonatomic-lo-hi.h>
>> ...
>>  pr_debug("Debug value of some register: %016llx\n", readq(addr));
>> 
>> However, lo_hi_readq() always returns __u64 data, while readq()
>> on x86_64 defines it as unsigned long. and thus compiler warns
>> about type mismatch, although they are both 64-bit on x86_64.
>> 
>> Convert readq() and writeq() on x86 to operate on deterministic
>> 64-bit type. The most of architectures in the kernel already are
>> using
>> either unsigned long long, or u64 type for readq() / writeq().
>> This change propagates consistency in that sense.
>> 
>> While this is not an issue per se, though if someone wants to address
>> it,
>> the anchor could be the commit
>> 
>>   797a796a13df ("asm-generic: architecture independent readq/writeq
>> for 32bit environment")
>> 
>> where non-atomic variants had been introduced.
>> 
>> Note, there are only few users of above pattern and they will not be
>> affected because they do cast returned value. The actual warning has
>> been issued on not-yet-upstreamed code.
>> 
>> Potentially we might get a new warnings if some 64-bit only code
>> assigns returned value to unsigned long type of variable. This is
>> assumed to be addressed on case-by-case basis.
>> 
>> Reported-by: lkp <lkp@...el.com>
>> Cc: Hitoshi Mitake <mitake@....info.waseda.ac.jp>
>> Cc: "Mehta, Sohil" <sohil.mehta@...el.com>
>> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
>> ---
>>  arch/x86/include/asm/io.h | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
>> index 95e948627fd0..365f5ba9222b 100644
>> --- a/arch/x86/include/asm/io.h
>> +++ b/arch/x86/include/asm/io.h
>> @@ -94,10 +94,10 @@ build_mmio_write(__writel, "l", unsigned int,
>> "r", )
>>  
>>  #ifdef CONFIG_X86_64
>>  
>> -build_mmio_read(readq, "q", unsigned long, "=r", :"memory")
>> -build_mmio_read(__readq, "q", unsigned long, "=r", )
>> -build_mmio_write(writeq, "q", unsigned long, "r", :"memory")
>> -build_mmio_write(__writeq, "q", unsigned long, "r", )
>> +build_mmio_read(readq, "q", unsigned long long, "=r", :"memory")
>> +build_mmio_read(__readq, "q", unsigned long long, "=r", )
>> +build_mmio_write(writeq, "q", unsigned long long, "r", :"memory")
>> +build_mmio_write(__writeq, "q", unsigned long long, "r", )
>>  
>>  #define readq_relaxed(a)	__readq(a)
>>  #define writeq_relaxed(v, a)	__writeq(v, a)
>
>The patch works for me:
>
>Tested-by: Sohil Mehta <sohil.mehta@...el.com>
>
>Sohil

Wouldn't simply u64 make more sense?
-- 
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