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Date:   Wed, 29 Nov 2017 12:57:35 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Greentime Hu <green.hu@...il.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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

On Wed, Nov 29, 2017 at 12:39 PM, Greentime Hu <green.hu@...il.com> wrote:
> 2017-11-29 17:25 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
>> On Wed, Nov 29, 2017 at 10:10 AM, Geert Uytterhoeven
>> <geert@...ux-m68k.org> wrote:
>>> 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.
>>
>> Right, my mistake.
>>
>
> Thanks to Arnd and Geert!
>
> How about this?
>
> choice
>         prompt "CPU type"
>         default CPU_N13
> config CPU_N15
>         bool "AndesCore N15"
>         select CPU_CACHE_NONALIASING
> config CPU_N13
>         bool "AndesCore N13"
>         select CPU_CACHE_NONALIASING if ANDES_PAGE_SIZE_8KB
> config CPU_N10
>         bool "AndesCore N10"
>         select CPU_CACHE_NONALIASING if ANDES_PAGE_SIZE_8KB
> config CPU_D15
>         bool "AndesCore D15"
>         select CPU_CACHE_NONALIASING
>         select HWZOL
> config CPU_D10
>         bool "AndesCore D10"
>         select CPU_CACHE_NONALIASING if ANDES_PAGE_SIZE_8KB
> endchoice

With a 'choice' statement this works, but I would consider that
suboptimal for another reason: now you cannot build a kernel that
works on e.g. both N13 and N15.

This is what we had on ARM a long time ago and on MIPS not so long
ago, but it's really a burden for testing and distribution once you get
support for more than a handful of machines supported in the mainline
kernel: If each CPU core is mutually incompatible with the other ones,
this means you likely end up having one defconfig per CPU core,
or even one defconfig per SoC or board.

I would always try to get the largest amount of hardware to work
in the same kernel concurrently.

One way of of this would be to define the "CPU type" as the minimum
supported CPU, e.g. selecting D15 would result in a kernel that
only works on D15, while selecting N15 would work on both N15 and
D15, and selecting D10 would work on both D10 and D15.

       Arnd

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