linux-kernel - Re: [PATCH v3] LoongArch/percpu: Simplify _percpu_read() and _percpu

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Message-ID: <361bf35ccf308d2fbce14f69f156cb42eef0f095.camel@xry111.site>
Date: Thu, 05 Sep 2024 20:09:35 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Uros Bizjak <ubizjak@...il.com>, Huacai Chen <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org, WANG Xuerui
	 <kernel@...0n.name>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3] LoongArch/percpu: Simplify _percpu_read() and
 _percpu_write()

On Thu, 2024-09-05 at 14:02 +0200, Uros Bizjak wrote:
> > If the input value is less than 0xff, then "& 0xff" is meaningless, if
> > the input value is more than 0xff, this conversion still cannot give a
> > correct result for the caller. So I think for all sizes it is enough
> > to just use "((unsigned long) val)".
> 
> This part is used to force unsigned extension, otherwise the compiler
> will use sign-extension of the possibly signed variable.

It's not relevant.  For example when size is 2 __pcpu_op_##size("stx")
is expanded to stx.h, and stx.h only stores the lower 16 bits of a
register into MEM[r21 + ptr], the high bits are ignored anyway.

Thus we can just have

+#define _percpu_write(size, _pcp, _val)					\
+do {									\
+	if (0) {		                                        \
+		typeof(_pcp) pto_tmp__;					\
+		pto_tmp__ = (_val);					\
+		(void)pto_tmp__;					\
+	}								\
+	__asm__ __volatile__(						\
+		__pcpu_op_##size("stx") "%[val], $r21, %[ptr]	\n"	\
+		:							\
+		: [val] "r"(_val), [ptr] "r"(&(_pcp))		\
+		: "memory");						\
+} while (0)

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University