Re: [PATCH v19 02/13] x86/setup: Use parse_crashkernel_high_low() to simplify code

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2021/12/30 0:51, Borislav Petkov wrote:
> On Wed, Dec 29, 2021 at 11:04:21PM +0800, Leizhen (ThunderTown) wrote:
>> Chen Zhou and I tried to share the code because of a suggestion. After so many
>> attempts, it doesn't seem to fit to make generic. Or maybe I haven't figured
>> out a good solution yet.
> 
> Well, you learned a very important lesson and the many attempts are not
> in vain: code sharing does not make sense in every case.
> 
>> I will put the patches that make arm64 support crashkernel...high,low to
>> the front, then the parse_crashkernel() unification patches. Even if the
>> second half of the patches is not ready for v5.18, the first half of the
>> patches is ready.
> 
> I think you should concentrate on the arm64 side which is, AFAICT, what
> you're trying to achieve.

Right, a patchset should focus on just one thing.

> 
> The "parse_crashkernel() unification" needs more thought because, as I
> said already, that doesn't make a whole lot of sense to me.

Yes, because it's not a functional improvement, it's not a performance optimization,
it's also not a fix for a known bug, it's just a programmer's artistic pursuit.

> 
> If you want to enforce the fact that "low" makes sense only when "high"
> is supplied, parse_crashkernel_high_low() is not the right thing to do.
> You need to have a *single* function which does all the parsing where
> you can decide what to do: "if high, parse low", "if no high supplied,
> ignore low" and so on.

I understand your proposal, but parse_crashkernel_high_low() is a cost-effective
and profitable change, it makes the current code a little clearer, and avoid passing
unnecessary parameters "system_ram" and "crash_base" when other architectures use
parse_crashkernel_{high|low}().

I actually followed your advice in the beginning to do "parse_crashkernel() and
parse_crashkernel_{high|low}() unification". But I found it's difficult and the
end result may not be as good as expected. So I introduced parse_crashkernel_high_low().

The parameter "system_ram" and "crash_base" of parse_crashkernel() is not need by
"crashkernel=X,[high,low]". And parameter "low_size" of parse_crashkernel_high_low()
is not need by "crashkernel=X[@offset]". The "parse_crashkernel() unification"
complicates things. For example, the parameter "crash_size" means "low or high" memory
size for "crashkernel=X[@offset]", but only means "high" memory size for "crashkernel=X,high".
So we'd better give it two names with union.

> 
> And if those are supported on certain architectures only, you can do
> ifdeffery...

I don't think so. These __init functions are small and architecture-independent, and do not
affect compilation of other architectures. There may be other architectures that use
it in the future, such as the current arm64.

> 
> But I think I already stated that I don't like such unifications which
> introduce unnecessary dependencies between architectures. Therefore, I
> won't accept them into x86 unless there's a strong compelling reason.
> Which I don't see ATM.

OK.

> 
> Thx.
> 

-- 
Regards,
  Zhen Lei