linux-kernel - Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument

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Date:   Wed, 8 Jan 2020 09:48:46 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Krzysztof Kozlowski <krzk@...nel.org>,
        Rich Felker <dalias@...c.org>,
        Jiri Slaby <jirislaby@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
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Subject: Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument

Hi Geert,

Le 08/01/2020 à 09:43, Geert Uytterhoeven a écrit :
> Hi Christophe,
> 
> On Wed, Jan 8, 2020 at 9:35 AM Christophe Leroy <christophe.leroy@....fr> wrote:
>> Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit :
>>> On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>>>> On Wed, Jan 8, 2020 at 9:07 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>>>>> On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
>>>>>> The ioread8/16/32() and others have inconsistent interface among the
>>>>>> architectures: some taking address as const, some not.
>>>>>>
>>>>>> It seems there is nothing really stopping all of them to take
>>>>>> pointer to const.
>>>>>
>>>>> Shouldn't all of them take const volatile __iomem pointers?
>>>>> It seems the "volatile" is missing from all but the implementations in
>>>>> include/asm-generic/io.h.
>>>>
>>>> As my "volatile" comment applies to iowrite*(), too, probably that should be
>>>> done in a separate patch.
>>>>
>>>> Hence with patches 1-5 squashed, and for patches 11-13:
>>>> Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>>
>>> I'll add to this one also changes to ioreadX_rep() and add another
>>> patch for volatile for reads and writes. I guess your review will be
>>> appreciated once more because of ioreadX_rep()
>>
>> volatile should really only be used where deemed necessary:
>>
>> https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html
>>
>> It is said: " ...  accessor functions might use volatile on
>> architectures where direct I/O memory access does work. Essentially,
>> each accessor call becomes a little critical section on its own and
>> ensures that the access happens as expected by the programmer."
> 
> That is exactly the use case here: all above are accessor functions.
> 
> Why would ioreadX() not need volatile, while readY() does?
> 

My point was: it might be necessary for some arches and not for others.

And as pointed by Arnd, the volatile is really only necessary for the 
dereference itself, should the arch use dereferencing.

So I guess the best would be to go in the other direction: remove 
volatile keyword wherever possible instead of adding it where it is not 
needed.

Christophe