linux-kernel - Re: [PATCH] riscv: fixup max_low_pfn with PFN

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Message-ID: <20190115161254.GA19081@infradead.org>
Date:   Tue, 15 Jan 2019 08:12:54 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>, palmer@...ive.com,
        aou@...s.berkeley.edu, linux-arch@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Guo Ren <ren_guo@...ky.com>, Mao Han <mao_han@...ky.com>
Subject: Re: [PATCH] riscv: fixup max_low_pfn with PFN_DOWN.

On Wed, Jan 16, 2019 at 12:10:00AM +0800, Guo Ren wrote:
> > >  	set_max_mapnr(PFN_DOWN(mem_size));
> > > -	max_low_pfn = memblock_end_of_DRAM();
> > > +	max_low_pfn = PFN_DOWN(memblock_end_of_DRAM());
> > 
> > I know it is used just above, but can we please just switch this
> > code to use >> PAGE_SHIFT instead of PFN_DOWN, which just horribly
> > obsfucates what is going on?
> ???
> #define PFN_DOWN(x)	((x) >> PAGE_SHIFT)
> 
> phys_addr_t __init_memblock memblock_end_of_DRAM(void)
> {
> 	int idx = memblock.memory.cnt - 1;
> 
> 	return (memblock.memory.regions[idx].base + memblock.memory.regions[idx].size);
> }
> 
> What's the problem? PFN_DOWN() couldn't be used with function call?

PFN_DOWN gives you the correct result.  But I think it actually
drastically reduces readability over just opencoding it.

> My patch just want to point out that max_low_pfn is PFN not size. In fact
> there is no error for running without my patch :P

No, I think your patch is correct.  I just wonder if we could make
the code easier to read.