Re: [PATCH] scsi: lpfc: use memcpy_toio instead of writeq

[Andy Shevchenko]

On Fri, Feb 23, 2018 at 7:09 PM, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Andy Shevchenko
>> Sent: 23 February 2018 16:51
>> On Fri, Feb 23, 2018 at 6:41 PM, David Laight <David.Laight@xxxxxxxxxx> 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