linux-kernel - Re: [PATCH v2 25/35] nds32: Build infrastructure

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Date:   Wed, 29 Nov 2017 10:10:39 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greentime Hu <green.hu@...il.com>,
        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" <linux-serial@...r.kernel.org>,
        Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v2 25/35] nds32: Build infrastructure

Hi Arnd,

On Wed, Nov 29, 2017 at 9:58 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Wed, Nov 29, 2017 at 9:39 AM, Greentime Hu <green.hu@...il.com> wrote:
>> 2017-11-27 22:21 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
>>> On Mon, Nov 27, 2017 at 1:28 PM, Greentime Hu <green.hu@...il.com> wrote:
>>>> diff --git a/arch/nds32/Kconfig.cpu b/arch/nds32/Kconfig.cpu
>>>> +config CPU_CACHE_NONALIASING
>>>> +       bool "Non-aliasing cache"
>>>> +       help
>>>> +         If this CPU is using VIPT data cache and its cache way size is larger
>>>> +         than page size, say N. If it is using PIPT data cache, say Y.
>>>> +
>>>> +         If unsure, say Y.
>>>
>>> Can you determine this from the CPU type?
>>
>> There is no cpu register to determine it. It also depeneds on page
>> size and way size however page size is configurable by software.
>> These codes are determined at compile time will be benefit to code
>> size and performance.
>> IMHO, I think it would be better to be determined here.
>
> I meant determining it at compile time from other Kconfig symbols,
> if that's possible. Do the CPU cores each have a fixed way-size?
> If they do, it could be done like
>
> menu "CPU selection"
>
> config CPU_N15
>       bool "AndesCore N15"
>       select CPU_CACHE_NONALIASING
>
> config CPU_N13
>       bool "AndesCore N15"
>       select CPU_CACHE_NONALIASING if PAGE_SIZE_16K
>
> ...
>
> endmenu
>
> and then you can use the same CPU_... symbols to make other decisions
> as well, e.g. CPU specific compiler optimizations.

Do you want to support multiple CPU types in a single kernel image
(I see no "choice" statement above)?
If yes, you may have a mix of aliasing and non-aliasing caches, so
you may want to invert the logic, and select CPU_CACHE_ALIASING
instead.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds