linux-kernel - Re: [PATCH] mips: Remove q-accessors from non-64bit platforms

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Date:   Fri, 21 Jun 2019 11:09:03 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@...ux-mips.org>
To:     Arnd Bergmann <arnd@...db.de>
cc:     Paul Burton <paul.burton@...s.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Serge Semin <Sergey.Semin@...latforms.ru>,
        "Vadim V . Vlasov" <vadim.vlasov@...latforms.ru>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mips: Remove q-accessors from non-64bit platforms

On Fri, 21 Jun 2019, Arnd Bergmann wrote:

> >  The use of 64-bit operations to access option's packet memory, which is
> > true SRAM, i.e. no side effects, is to improve throughput only and there's
> > no need for atomicity here nor also any kind of barriers, except at the
> > conclusion.  Splitting 64-bit accesses into 32-bit halves in software
> > would not be a functional error here.
> 
> The other property of packet memory and similar things is that you
> basically want memcpy()-behavior with no byteswaps. This is one
> of the few cases in which __raw_readq() is actually the right accessor
> in (mostly) portable code.

 Correct, but we're missing an `__raw_readq_relaxed', etc. interface and 
having additional barriers applied on every access would hit performance 
very badly; in fact even the barriers `*_relaxed' accessors imply would 
best be removed in this use (which is why defza.c uses `readw_o' vs 
`readw_u', etc. internally), but after all the struggles over the years 
for weakly ordered internal APIs x86 people are so averse to I'm not sure 
if I want to start another one.  We can get away with `readq_relaxed' in 
this use though as all the systems this device can be used with are 
little-endian as is TURBOchannel, so no byte-swapping will ever actually 
occur.

  Maciej