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

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Message-ID: <CAEbi=3dfN6zpcNUpwbsPC_N_6w66=p8J=z8dUxHvyf11KtRQKw@mail.gmail.com>
Date:   Wed, 20 Dec 2017 10:34:45 +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>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v4 25/36] nds32: Miscellaneous header files

2017-12-19 17:54 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
> 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.
>

Thanks.
Got it. I will add a one-line comment in delay.h