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Date:   Thu, 9 Nov 2017 11:33:10 +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 <vincentc@...estech.com>
Subject: Re: [PATCH 26/31] nds32: Build infrastructure

On Thu, Nov 9, 2017 at 10:02 AM, Greentime Hu <green.hu@...il.com> wrote:
> 2017-11-08 18:16 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
>> On Wed, Nov 8, 2017 at 6:55 AM, Greentime Hu <green.hu@...il.com> wrote:

>>> +config GENERIC_CALIBRATE_DELAY
>>> +       def_bool y
>>
>> It's better to avoid the delay loop completely and skip the calibration,
>> if your hardware allows.
>
> Thanks.
> Do you mean that this config should be def_bool n?
> why? Almost all arch enable it.

It depends on what your hardware can do. If you have a way to see how much
time has passed that is guaranteed to be reliable on all machines, then
use that instead.

On a lot of architectures, it's not possible, so they have to fall back to using
the delay loop.

>>> +config ALIGNMENT_TRAP
>>> +       tristate "Kernel support unaligned access handling"
>>> +       default y
>>> +       help
>>> +         Andes processors cannot fetch/store information which is not
>>> +         naturally aligned on the bus, i.e., a 4 byte fetch must start at an
>>> +         address divisible by 4. On 32-bit Andes processors, these non-aligned
>>> +         fetch/store instructions will be emulated in software if you say
>>> +         here, which has a severe performance impact. This is necessary for
>>> +         correct operation of some network protocols. With an IP-only
>>> +         configuration it is safe to say N, otherwise say Y.
>>
>> Which network protocols are you referring to?
>
> I will modify these descriptions. It was written by someone I don't know. :p
> This case only happened when I found is compiler code gen issue or
> wrong pointer usage.

Ok, should it also be 'default n' then?

>>> +config HIGHMEM
>>> +       bool "High Memory Support"
>>> +       depends on MMU && CPU_CACHE_NONALIASING
>>> +       help
>>> +         The address space of Andes processors is only 4 Gigabytes large
>>> +         and it has to accommodate user address space, kernel address
>>> +         space as well as some memory mapped IO. That means that, if you
>>> +         have a large amount of physical memory and/or IO, not all of the
>>> +         memory can be "permanently mapped" by the kernel. The physical
>>> +         memory that is not permanently mapped is called "high memory".
>>> +
>>> +         Depending on the selected kernel/user memory split, minimum
>>> +         vmalloc space and actual amount of RAM, you may not need this
>>> +         option which should result in a slightly faster kernel.
>>> +
>>> +         If unsure, say N.
>>
>> Generally speaking, highmem support is a mess, and it's better to avoid it.
>>
>> I see that the two device tree files you have list 1GB of memory. Do you think
>> that is a common configuration for actual products? Do you expect any to
>> have more than 1GB (or more than 4GB) in the future, or is that the upper
>> end of the scale?
>>
>> If 1GB is a reasonable upper bound, then you could change the vmsplit
>> to give slightly less address space to user space and have 1GB of direct-mapped
>> kernel memory plus 256MB of vmalloc space reserved for the kernel,
>> and completely avoid highmem.
>
> Thanks.
> We do realy use 1GB ram in some products.
> We also verify CONFIG_HIGHMEM with LTP too.
> It seems fine but I will study vmsplit to see if we should use it.

For the 1GB configuration, something like ARM's CONFIG_VMSPLIT_3G_OPT
should be optimal, it will result in better performance because it allows you
to completely turn off CONFIG_HIGHMEM. The reason we don't always
use it on ARM is that traditionally we had the 3GB vmsplit, and some
applications
might rely on having the exact amount of available address space that they
expect. For a new architecture that should be less of a problem.

The interesting case is what happens if you have machines with 1.5GB or
or more physical RAM. You can obviously have another vmsplit configuration
for those, but at some point going to highmem is better than limiting the
user address space.

Ideally 1.5GB is the point where you start using a 64-bit CPU, but of course
that is not something you have available at the moment.

>>> +config MEMORY_START
>>> +       hex "Physical memory start address"
>>> +       default "0x00000000"
>>> +       help
>>> +         Physical memory start address, you may modify it if it is porting to
>>> +         a new SoC with different start address.
>>> +endmenu
>>
>> On ARM, we found options like this to be rather problematic since it prevents
>> you from running the same kernel on boards that are otherwise compatible.
>>
>> If the architecture easily allows the memory to start at address 0, could
>> you require this address for all SoCs that want to run Linux, and get
>> rid of the compile-time option?
>
> Thanks.
> The reason we need this config is because we need to define PHYS_OFFSET.
> #define PHYS_OFFSET     (CONFIG_MEMORY_START)
>
> It needs to be set in compile-time. I don't know how to get rid of it.

PHYS_OFFSET doesn't have to be a constant, a lot of architectures make
the __va()/__pa() and related functions use a variable for the offset.
This is also useful to implement KASLR, and booting the kernel from
a random physical address.

My actual suggestion however was to just mandate that PHYS_OFFSET
is always zero for your architecture, and not support any other value.
This is easy as long as you don't have existing hardware that would
break.

        Arnd

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