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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Fri, 23 Feb 2018 19:12:44 +0200
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Subject: Re: [PATCH] scsi: lpfc: use memcpy_toio instead of writeq
To:     David Laight <David.Laight@aculab.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        James Smart <james.smart@broadcom.com>,
        Dick Kennedy <dick.kennedy@broadcom.com>,
        "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Hannes Reinecke <hare@suse.com>,
        Johannes Thumshirn <jthumshirn@suse.de>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Fri, Feb 23, 2018 at 7:09 PM, David Laight <David.Laight@aculab.com> wrote:
> From: Andy Shevchenko
>> Sent: 23 February 2018 16:51
>> On Fri, Feb 23, 2018 at 6:41 PM, David Laight <David.Laight@aculab.com> wrote:


>> The side-effect I referred previously is about tails, i.e. unaligned
>> bytes are transferred in portions
>> like
>>   7 on 64-bit will be  4 + 2 + 1,
>>   5 = 4 + 1
>
> on 64bit memcpy() is allowed to do:
>         (long *)(tgt+len)[-1] = (long *)(src+len)[-1];
>         rep_movsq(tgt, src, len >> 3);
> provided the length is at least 8.
>
> The misaligned PCIe transfer generates a single TLP covering 12 bytes with the
> relevant byte enables set for the first and last 32bit words.

But is it guaranteed on any type of bus?
memcpy_toio() is a generic helper, so, first of all we need to be sure
what CPU on its side does, this is I think is pretty straight forward
since it's all written in asm for 64-bit case.
So, what about buses?

-- 
With Best Regards,
Andy Shevchenko