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From: Arnd Bergmann <arnd@arndb.de>
Date: Tue, 19 Dec 2017 10:54:59 +0100
Message-ID: <CAK8P3a3Ofczq1DrQEcEcP1fZrgyeOLpFDwgd7uMZ4H0NpHs+wg@mail.gmail.com>
Subject: Re: [PATCH v4 25/36] nds32: Miscellaneous header files
To: Greentime Hu <green.hu@gmail.com>
Cc: Greentime <greentime@andestech.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jason Cooper <jason@lakedaemon.net>,
        Marc Zyngier <marc.zyngier@arm.com>, Rob Herring <robh+dt@kernel.org>,
        Networking <netdev@vger.kernel.org>,
        Vincent Chen <deanbo422@gmail.com>, DTML <devicetree@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>, David Howells <dhowells@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        linux-serial@vger.kernel.org,
        Geert Uytterhoeven <geert.uytterhoeven@gmail.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>, Greg KH <greg@kroah.com>,
        Guo Ren <ren_guo@c-sky.com>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        Vincent Chen <vincentc@andestech.com>
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On Tue, Dec 19, 2017 at 6:34 AM, Greentime Hu <green.hu@gmail.com> wrote:
> Hi, Arnd:
>
> 2017-12-18 19:13 GMT+08:00 Arnd Bergmann <arnd@arndb.de>:
>> On Mon, Dec 18, 2017 at 7:46 AM, Greentime Hu <green.hu@gmail.com> wrote:
>>> From: Greentime Hu <greentime@andestech.com>
>>>
>>> This patch introduces some miscellaneous header files.
>>
>>> +static inline void __delay(unsigned long loops)
>>> +{
>>> +       __asm__ __volatile__(".align 2\n"
>>> +                            "1:\n"
>>> +                            "\taddi\t%0, %0, -1\n"
>>> +                            "\tbgtz\t%0, 1b\n"
>>> +                            :"=r"(loops)
>>> +                            :"0"(loops));
>>> +}
>>> +
>>> +static inline void __udelay(unsigned long usecs, unsigned long lpj)
>>> +{
>>> +       usecs *= (unsigned long)(((0x8000000000000000ULL / (500000 / HZ)) +
>>> +                                 0x80000000ULL) >> 32);
>>> +       usecs = (unsigned long)(((unsigned long long)usecs * lpj) >> 32);
>>> +       __delay(usecs);
>>> +}
>>
>> Do you have a reliable clocksource that you can read here instead of doing the
>> loop? It's generally preferred to have an accurate delay if at all possible, the
>> delay loop calibration is only for those architectures that don't have any
>> way to observe how much time has passed accurately.
>>
>
> We currently only have atcpit100 as clocksource but it is an IP of  SoC.
> These delay API will be unavailable if we changed to another SoC
> unless all these timer driver provided the same APIs.
> It may suffer our customers if they forget to port these APIs in their
> timer drivers when they try to use nds32 in the first beginning.

Ok, thanks for the clarification.

> Or maybe I can use a CONFIG_USE_ACCURATE_DELAY to keep these 2
> implementions for these purposes?

I'd just add a one-line comment in delay.h to explain that there is no
cycle counter in the CPU.

       Arnd