Re: [PATCH 00/13] MIPS: Convert Ingenic to a generic board

From: Paul Cercueil <paul@crapouillou.net>
To: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: "Zhou Yanjie" <zhouyanjie@wanyeetech.com>,
	"Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
	"Paul Burton" <paulburton@kernel.org>,
	"Krzysztof Kozlowski" <krzk@kernel.org>,
	od@zcrc.me, linux-kernel@vger.kernel.org,
	linux-mips@vger.kernel.org, 漆鹏振 <aric.pzqi@ingenic.com>,
	dongsheng.qiu@ingenic.com, rick.tyliu@ingenic.com,
	yanfei.li@ingenic.com, xuwanhao@wanyeetech.com
Subject: Re: [PATCH 00/13] MIPS: Convert Ingenic to a generic board
Date: Sat, 22 Aug 2020 15:17:49 +0200	[thread overview]
Message-ID: <PLVGFQ.TBOPD0BSHD5L1@crapouillou.net> (raw)
In-Reply-To: <alpine.LFD.2.21.2008220310130.3460685@eddie.linux-mips.org>

Le sam. 22 août 2020 à 3:29, Maciej W. Rozycki <macro@linux-mips.org> 
a écrit :
> Hi Paul,
> 
>>  > FAOD <cpu-feature-overrides.h> is not a hack, but an optimisation 
>> measure
>>  > so that features known to be hardwired for a given machine/CPU do 
>> not have
>>  > to be dynamically queried every time referred.  In some cases 
>> that results
>>  > in large portions of code being optimised away by the compiler as 
>> well.
>> 
>>  Fair enough. Bloat-o-meter reports about ~100 KiB saved when that 
>> file is
>>  present. But we can't use it in a generic kernel, unfortunately.
> 
>  Well, run-time patching might be an alternative to get the best of 
> both
> worlds, but someone would have to reimplement our feature selection 
> system
> to use it.

Would run-time patching allow to drop dead code?

>>  > The hardcoded value for a feature defined in 
>> <cpu-feature-overrides.h>
>>  > always has to be the same as one in the corresponding bit of the 
>> `options'
>>  > member of `struct cpuinfo_mips', in this case MIPS_CPU_TLBINV.
>> 
>>  In theory yes, in practice the CPU detection code is lagging 
>> behind...
> 
>  I wasn't aware of that.  In that case it has been a design abuse 
> which
> has been missed by the maintainer when accepting patches.  It used to 
> be
> the case that run-time detection was accurate and overrides were 
> rather
> lazily added.
> 
>  Also I note Ingenic must have had a CPU erratum if our 
> `decode_configs'
> doesn't just work, as the interpretation of CP0.Config[5:0] registers 
> has
> been architectural and mandatory, and that for a reason.  It's only 
> legacy
> MIPS I-IV processors that should require special attention here.

Yes, Ingenic CPUs have a few bloopers here and there...

-Paul