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

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Date:   Tue, 19 Dec 2017 13:34:47 +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

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