linux-kernel - Re: [PATCH] scsi: lpfc: use memcpy

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Message-ID: <CAK8P3a1gsKOL=2epyvwnWe5LbZ0L+vHVuM5zfz3gS25ptQ2UrQ@mail.gmail.com>
Date:   Mon, 26 Feb 2018 10:03:58 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     James Smart <james.smart@...adcom.com>,
        Dick Kennedy <dick.kennedy@...adcom.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Hannes Reinecke <hare@...e.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scsi: lpfc: use memcpy_toio instead of writeq

On Sun, Feb 25, 2018 at 11:02 AM, Johannes Thumshirn <jthumshirn@...e.de> wrote:
> Arnd Bergmann <arnd@...db.de> writes:
>> 32-bit architectures generally cannot use writeq(), so we now get a build
>> failure for the lpfc driver:
>>
>> drivers/scsi/lpfc/lpfc_sli.c: In function 'lpfc_sli4_wq_put':
>> drivers/scsi/lpfc/lpfc_sli.c:145:4: error: implicit declaration of function 'writeq'; did you mean 'writeb'? [-Werror=implicit-function-declaration]
> Hi Arnd,
> why can't we use the writeq() from 'io-64-nonatomic-lo-hi.h'? I always
> thought these are compat versions for 32 Bit archs and even asked James
> to do so, what's why he did the change in the first place.

That could work as well, if someone can figure out what the correct incantation
is that works on big-endian 32-bit architectures. I think the simplest
version that
works everywhere would be

lo_hi_writeq(__le32_to_cpup(__le32 __force *)p) |
(u64)__le32_to_cpup(__le32 __force *)p +1) << 32));

but this is ugly as hell and makes my head spin. I definitely would't want that
applied without being tested properly first on a variety of architectures.

There are three variants that I'd prefer over that:

- use memcpy_toio() and change the x86 implementation to guarantee aligned
  word aligned accesses on the output if at all possible (which seems to be
  a good idea anyway)
- use a loop around __raw_writeq()/__raw_writel() depending on CONFIG_64BIT
- add generic memcpy_toio_32() and memcpy_toio_64() helpers in linux/io.h
  and use those.

       Arnd