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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 7 Sep 2018 10:14:38 +0200
Message-ID: <CAK8P3a2K_ecCQEs840hQMrjsf4H8hHxS+HtM2TtPDYpRu8-LeQ@mail.gmail.com>
Subject: Re: [PATCH V3 06/26] csky: Cache and TLB routines
To:     Guo Ren <ren_guo@c-sky.com>
Cc:     linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Jason Cooper <jason@lakedaemon.net>,
        c-sky_gcc_upstream@c-sky.com, gnu-csky@mentor.com,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        wbx@uclibc-ng.org, Greentime Hu <green.hu@gmail.com>
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On Fri, Sep 7, 2018 at 5:04 AM Guo Ren <ren_guo@c-sky.com> wrote:
>
> On Thu, Sep 06, 2018 at 04:31:16PM +0200, Arnd Bergmann wrote:
> > On Wed, Sep 5, 2018 at 2:08 PM Guo Ren <ren_guo@c-sky.com> wrote:
> >
> > Can you describe how C-Sky hardware implements MMIO?
> Our mmio is uncachable and strong-order address, so there is no need
> barriers for access these io addr.
>
>  #define ioremap_wc ioremap_nocache
>  #define ioremap_wt ioremap_nocache
>
> Current ioremap_wc and ioremap_wt implementation are too simple and
> we'll improve it in future.
>
> > In particular:
> >
> > - Is a read from uncached memory always serialized with DMA, and with
> >   other CPUs doing MMIO access to a different address?
> CPU use ld.w to get data from uncached strong order memory.
> Other CPUs use the same mmio vaddr to access the uncachable strong order
> memory paddr.

Ok, but what about the DMA? The most common requirement for
serialization here is with a DMA transfer, where you first write
into a buffer in memory, then write to an MMIO register to trigger
a DMA-load, and then the device reads the data from memory.
Without a barrier before the MMIO, the data may still be in a
store queue of the CPU, and the DMA gets stale data.

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.

> > - 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.

        Arnd