linux-kernel - Re: [PATCH V3 06/26] csky: Cache and TLB routines

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Date:   Fri, 7 Sep 2018 16:13:35 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <ren_guo@...ky.com>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Jason Cooper <jason@...edaemon.net>,
        c-sky_gcc_upstream@...ky.com, gnu-csky@...tor.com,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        wbx@...ibc-ng.org, Greentime Hu <green.hu@...il.com>
Subject: Re: [PATCH V3 06/26] csky: Cache and TLB routines

On Fri, Sep 7, 2018 at 2:55 PM Guo Ren <ren_guo@...ky.com> wrote:
>
> On Fri, Sep 07, 2018 at 10:14:38AM +0200, Arnd Bergmann wrote:
> > On Fri, Sep 7, 2018 at 5:04 AM Guo Ren <ren_guo@...ky.com> wrote:
> > > On Thu, Sep 06, 2018 at 04:31:16PM +0200, Arnd Bergmann wrote:
> > Similarly, an MMIO read may be used to see if a DMA has completed
> > and the device register tells you that the DMA has left the device,
> > but without a barrier, the CPU may have prefetched the DMA
> > data while waiting for the MMIO-read to complete. The __io_ar()
> > barrier() in asm-generic/io.h prevents the compiler from reordering
> > the two reads, but if an weakly ordered read (in coherent DMA buffer)
> > can bypass a strongly ordered read (MMIO), then it's still still
> > broken.
> __io_ar() barrier()? not rmb() ?! I've defined the rmb in asm/barrier, So
> I got rmb() here not barrier().
>
> Only __io_br() is barrier().

Ah right, I misremembered the defaults. It's probably ok then.

> > > > - How does endianess work? Are there any buses that flip bytes around
> > > >   when running big-endian, or do you always do that in software?
> > > Currently we only support little-endian and soc will follow it.
> >
> > Ok, that makes it easier. If you think that you won't even need big-endian
> > support in the long run, you could also remove your asm/byteorder.h
> > header. If you're not sure, it doesn't hurt to keep it of course.
> Em... I'm not sure, so let me keep it for a while.

Ok. I think overall the trend is to be little-endian only for most
architectures: powerpc64 moved from big-endian only to little-endian
by default, ARM rarely uses big-endian (basically only for legacy
applications ported from BE MIPS or ppc), and all new architectures
we added in the last years are little-endian (OpenRISC being the
main exception).

     Arnd