Re: [PATCH] PPC/PPC64: Introduce CPU_HAS_FEATURE() macro

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@osdl.org>,
	Tom Rini <trini@kernel.crashing.org>,
	Linux Kernel list <linux-kernel@vger.kernel.org>,
	linuxppc-dev list <linuxppc-dev@ozlabs.org>,
	Paul Mackerras <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linuxppc64-dev <linuxppc64-dev@ozlabs.org>
Subject: Re: [PATCH] PPC/PPC64: Introduce CPU_HAS_FEATURE() macro
Date: Sat, 05 Feb 2005 10:49:49 +1100	[thread overview]
Message-ID: <1107560989.2189.119.camel@gaston> (raw)
In-Reply-To: <200502041336.59892.arnd@arndb.de>

On Fri, 2005-02-04 at 13:36 +0100, Arnd Bergmann wrote:
> On Freedag 04 Februar 2005 08:22, Olof Johansson wrote:
> > It's getting pretty old to have see and type cur_cpu_spec->cpu_features
> > & CPU_FTR_<feature>, when a shorter and less TLA-ridden macro is more
> > readable.
> > 
> > This also takes care of the differences between PPC and PPC64 cpu
> > features for the common code; most places in PPC could be replaced with
> > the macro as well.
> 
> I have a somewhat similar patch that does the same to the
> systemcfg->platform checks. I'm not sure if we should use the same inline
> function for both checks, but I do think that they should be used in a
> similar way, e.g. CPU_HAS_FEATURE(x) and PLATFORM_HAS_FEATURE(x).

Note that I would prefer cpu_has_feature(), it doesn't strictly have to
be a macro and has function semantics anyway.

> My implementation of the platform checks tries to be extra clever by turning
> runtime checks into compile time checks if possible. This reduces code size
> and in some cases execution speed. It can also be used to replace compile
> time checks, i.e. it allows us to write
> 
> static inline unsigned int readl(const volatile void __iomem *addr)
> {
> 	if (platform_is(PLATFORM_PPC_ISERIES))
> 		return iSeries_readl(addr);
> 	if (platform_possible(PLATFORM_PPC_PSERIES))
> 		return eeh_readl(addr);
> 	return in_le32();
> }
> 
> which will always result in the shortest code for any combination of
> CONFIG_PPC_ISERIES, CONFIG_PPC_PSERIES and the other platforms.

That's a good idea !

> The required code for this is roughly
> 
> enum {
> 	PPC64_PLATFORM_POSSIBLE =
> #ifdef CONFIG_PPC_ISERIES
> 		PLATFORM_ISERIES |
> #endif
> #ifdef CONFIG_PPC_PSERIES
> 		PLATFORM_PSERIES |
> #endif
> #ifdef CONFIG_PPC_PSERIES
> 		PLATFORM_PSERIES_LPAR |
> #endif
> #ifdef CONFIG_PPC_POWERMAC
> 		PLATFORM_POWERMAC |
> #endif
> #ifdef CONFIG_PPC_MAPLE
> 		PLATFORM_MAPLE |
> #endif
> 		0,
> 	PPC64_PLATFORM_ONLY =
> #ifdef CONFIG_PPC_ISERIES
> 		PLATFORM_ISERIES &
> #endif
> #ifdef CONFIG_PPC_PSERIES
> 		PLATFORM_PSERIES &
> #endif
> #ifdef CONFIG_PPC_POWERMAC
> 		PLATFORM_POWERMAC &
> #endif
> #ifdef CONFIG_PPC_MAPLE
> 		PLATFORM_MAPLE &
> #endif
> 		-1ul,
> };
> 
> static inline platform_is(unsigned long platform)
> {
> 	return ((PPC64_PLATFORM_ONLY & platform) 
> 	 || (PPC64_PLATFORM_POSSIBLE & platform & systemcfg->platform));
> }
> 
> static inline platform_possible(unsigned long platform)
> {
> 	reutrn !!(PPC64_PLATFORM_POSSIBLE & platform);
> }
> 
> The same stuff is obviously possible for cur_cpu_spec->cpu_features as well.
> Do you think that it will help there?
> 
> 	Arnd <><
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-- 
Benjamin Herrenschmidt <benh@kernel.crashing.org>