linux-kernel - Re: [PATCH] LoongArch: Update the flush cache policy

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Message-ID: <cbd6ed9d5be1d7112d69117a72e0cb0081f9b64b.camel@xry111.site>
Date: Mon, 06 May 2024 18:17:51 +0800
From: Xi Ruoyao <xry111@...111.site>
To: lijun <lijun01@...inos.cn>, chenhuacai@...nel.org, kernel@...0n.name, 
	lvjianmin@...ngson.cn, dongbiao@...ngson.cn, zhangbaoqi@...ngson.cn
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: Update the flush cache policy

On Mon, 2024-05-06 at 18:08 +0800, lijun wrote:
> volatile prevents compiler optimization by allowing the compiler
> 
>   to reread the address value of addr every time

But why is this ever needed?  What's wrong if the compiler optimizes it?

If the problem is the compiler may optimize it to cdesc->ways * 3 *
cdesc->sets * cdesc->linesz, unknowing cdesc->ways etc may magically
change, you should use READ_ONCE(cdesc->ways) etc.

I.e. use READ_ONCE on the expression which may magically change, instead
of hacking addr.  addr won't magically change.

> 在 2024/5/6 17:28, Xi Ruoyao 写道:
> > On Mon, 2024-05-06 at 17:24 +0800, Li Jun wrote:
> > > fix when LoongArch s3 resume, Can't find image information
> > > 
> > > Signed-off-by: Li Jun <lijun01@...inos.cn>
> > > Signed-off-by: Baoqi Zhang <zhangbaoqi@...ngson.cn>
> > > Signed-off-by: Jianmin Lv <lvjianmin@...ngson.cn>
> > > Signed-off-by: Biao Dong <dongbiao@...ngson.cn>
> > > ---
> > >   arch/loongarch/mm/cache.c | 24 +++++++++++++++++++++++-
> > >   1 file changed, 23 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/loongarch/mm/cache.c b/arch/loongarch/mm/cache.c
> > > index 6be04d36ca07..52872fa0e5d8 100644
> > > --- a/arch/loongarch/mm/cache.c
> > > +++ b/arch/loongarch/mm/cache.c
> > > @@ -63,6 +63,28 @@ static void flush_cache_leaf(unsigned int leaf)
> > >   	} while (--nr_nodes > 0);
> > >   }
> > >   
> > > +static void flush_cache_last_level(unsigned int leaf)
> > > +{
> > > +	u64 addr;
> > > +	int i, j, nr_nodes, way_size;
> > > +	struct cache_desc *cdesc = current_cpu_data.cache_leaves
> > > +
> > > leaf;
> > > +
> > > +	nr_nodes = loongson_sysconf.nr_nodes;
> > > +
> > > +	addr = CSR_DMW1_BASE;
> > > +	iocsr_write32(0x1, 0x280);
> > > +	way_size = cdesc->sets * cdesc->linesz;
> > > +	do {
> > > +		for (i = 0; i < (cdesc->ways * 3); i++) {
> > > +			for (j = 0; j < (cdesc->sets); j++) {
> > > +				*(volatile u32 *)addr;
> > ??? what does this line do?
> > 
> 

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