linux-kernel - Re: [PATCH v4 25/36] nds32: Miscellaneous header files

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Date:   Tue, 19 Dec 2017 10:54:59 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Greentime Hu <green.hu@...il.com>
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>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v4 25/36] nds32: Miscellaneous header files

On Tue, Dec 19, 2017 at 6:34 AM, Greentime Hu <green.hu@...il.com> wrote:
> Hi, Arnd:
>
> 2017-12-18 19:13 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
>> On Mon, Dec 18, 2017 at 7:46 AM, Greentime Hu <green.hu@...il.com> wrote:
>>> From: Greentime Hu <greentime@...estech.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