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

[David Laight]

From: Andy Shevchenko
> Sent: 23 February 2018 17:13
> To: David Laight
> Cc: Arnd Bergmann; James Smart; Dick Kennedy; James E.J. Bottomley; Martin K. Petersen; Hannes
> Reinecke; Johannes Thumshirn; linux-scsi@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] scsi: lpfc: use memcpy_toio instead of writeq
> 
> 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.

I've just done a compile test, on x86-64 memcpy_toio() generates a
call to memcpy() (checked with objdump -dr).
That is on a system running a 4.14 kernel, so is probably using the system
headers from that release.
I'd need to do a run-time test on a newer system verify what the PCIe slave
sees - but I changed our driver to do its own copy loops in order to avoid
byte transfers some time ago.

FWIW I was originally doing copy_to/from_user() directly to PCIe memory addresses!

On x86 'memory' on devices can always be accesses by simple instructions.
Hardware 'IO' addresses are not valid for memcpy_to/fromio().

	David