linux-kernel - Re: [PATCH v6 16/36] nds32: DMA mapping API

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Message-ID: <CAEbi=3cifP5owmV0qPoC0_7z6NMi56tAqM+C9JdbqjzMTAUWig@mail.gmail.com>
Date:   Thu, 25 Jan 2018 21:48:01 +0800
From:   Greentime Hu <green.hu@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greentime <greentime@...estech.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        Vincent Chen <deanbo422@...il.com>,
        DTML <devicetree@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-serial@...r.kernel.org,
        Geert Uytterhoeven <geert.uytterhoeven@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Rutland <mark.rutland@....com>, Greg KH <greg@...ah.com>,
        Guo Ren <ren_guo@...ky.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        David Miller <davem@...emloft.net>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v6 16/36] nds32: DMA mapping API

Hi, Arnd:

2018-01-25 18:42 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
> On Thu, Jan 25, 2018 at 4:45 AM, Greentime Hu <green.hu@...il.com> wrote:
>> 2018-01-24 19:36 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
>>> On Tue, Jan 23, 2018 at 12:52 PM, Greentime Hu <green.hu@...il.com> wrote:
>>>> 2018-01-23 16:23 GMT+08:00 Greentime Hu <green.hu@...il.com>:
>>>>> 2018-01-18 18:26 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
>>>>>> On Mon, Jan 15, 2018 at 6:53 AM, Greentime Hu <green.hu@...il.com> wrote:
>>>
>>> That looks reasonable enough, but it does depend on a number of factors,
>>> and the dma-mapping.h implementation is not just about cache flushes.
>>>
>>> As I don't know the microarchitecture, can you answer these questions:
>>>
>>> - are caches always write-back, or could they be write-through?
>> Yes, we can config it to write-back or write-through.
>
> Ok. If a WT-cache is common enough, you could optimize for that
> case by skipping the explicit writeback here and just doing a synchronizing
> instruction. Usually if the cache is configurable, one would pick the
> writeback option though, so it's probably not important.

Thank you for this suggestion.
We have optimized in cpu_dcache_wb_range() and it will be called from
cpu_dma_wb_range().
It will do nothing if it is a write-through config cache.

>>> - is the CPU physical address always the same as the address visible to the
>>>   device?
>> Yes, it is always the same unless the CPU uses local memory. The
>> physical address of local memory will overlap the original bus
>> address.
>> I think the local memory case can be ignored because we don't use it for now.
>
> Ok, makes sense.
>
>>> - are there devices that can only see a subset of the physical memory?
>> All devices are able to see the whole physical memory in our current
>> SoC, but I think other SoC may support such kind of HW behavior.
>
> This is one area that might need a more complex implementation then,
> depending on what devices are used in other SoCs. For network or
> storage devices, it's usually sufficient to configure a DMA mask
> from the "dma-ranges" property of the parent bus in the device tree,
> the kernel code will then use bounce buffers.
>
> For other types of drivers, using the streaming DMA interfaces
> can require using the swiotlb helper that performs the bounce
> buffering at in place of the cache operations. With a bit of luck,
> you won't ever need to worry about it, just mentioning it here in
> case you run into that problem later.
>
> The consistent_sync() implementaiton you showed earlier should be
> good enough then.  With that change,
>
> Acked-by: Arnd Bergmann <arnd@...db.de>

Thank you. :)