Re: [PATCH] riscv: fixup max_low_pfn with PFN_DOWN.

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On Tue, 15 Jan 2019 17:07:38 PST (-0800), guoren@xxxxxxxxxx wrote:
Hi Christoph,

I use PFN_DOWN() every where as possible and seems it's a habit
problem. So let risc-v maintainer to choose "PFN_DOW()" or
">> PAGE_SHIFT".

Also the same with "end_of_DRAM & max_low_pfn".

PFN_DOWN makes sense to me, as that's what we're trying to do here (round a physical address down to page frame number). Am a I misunderstanding something?


Best Regards
 Guo Ren

On Tue, Jan 15, 2019 at 08:12:54AM -0800, Christoph Hellwig wrote:
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.



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