linux-kernel - Re: About commit "io: change inX() to have their own IO barrier overrides"

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Date:   Mon, 2 Mar 2020 12:35:17 +0000
From:   John Garry <john.garry@...wei.com>
To:     Sinan Kaya <okaya@...nel.org>, Arnd Bergmann <arnd@...db.de>
CC:     "xuwei (O)" <xuwei5@...ilicon.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>
Subject: Re: About commit "io: change inX() to have their own IO barrier
 overrides"

Hi Sinan,

Thanks for getting back to me.

> On 2/28/2020 4:52 AM, John Garry wrote:
>> About the commit in the $subject 87fe2d543f81, would there be any
>> specific reason why the logic pio versions of these functions did not
>> get the same treatment 

In fact, your changes and the logic PIO changes went in at the same time.

or should not? I'm talking about lib/logic_pio.c
>> here - commit 031e3601869c ("lib: Add generic PIO mapping method")
>> introduced this.
>>
>> In fact, logic pio will override these for arm64 with the vanilla
>> defconfig these days.
> 
> We only looked at inX()/inY() and readX()/writeX() API because the
> semantics of these API are defined in the kernel documentation.

Could we consider adding __io_pbr() et al to the kernel Documentation? I 
couldn't find them and I had to rely on checking 64e2c67738 ("io: define 
several IO & PIO barrier types for the asm-generic version") commit 
message to find the definition.

> We looked at how to generalize this so that there is a uniform
> behavior across different architectures.
> 
> Is logic PIO subject to ordering issues?

Well the point is that we're still concerned here with using 
readX/writeX for MMIO-based IO port accesses, see *** from logic_pio.c:

#define BUILD_LOGIC_IO(bw, type)					
type logic_in##bw(unsigned long addr)					
{									
	type ret = (type)~0;						
	if (addr < MMIO_UPPER_LIMIT) {					
		ret = read##bw(PCI_IOBASE + addr); ***	
	} else if (addr >= MMIO_UPPER_LIMIT && addr < IO_SPACE_LIMIT) {
		struct logic_pio_hwaddr *entry = find_io_range(addr);	
									
		if (entry)						
			ret = entry->ops->in(entry->hostdata,		
					addr, sizeof(type));		
		else							
			WARN_ON_ONCE(1);				
	}								
	return ret;							
}		

 > How is the behavior on different architectures?

So today only ARM64 uses it for this relevant code, above. But maybe 
others in future will want to use it - any arch without native IO port 
access is a candidate.

> 
> As long as the expectations are set, I see no reason why it shouldn't
> but, I'll let Arnd comment on it too.

ok, so it looks reasonable consider replicating your change for ***, above.

Thanks,
John