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From:   Logan Gunthorpe <logang@deltatee.com>
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Date:   Fri, 29 Mar 2019 15:32:07 -0600
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Subject: Re: [PATCH] x86/calgary: fix bitcast type warnings
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On 2019-03-29 3:19 p.m., Jann Horn wrote:
>>> Can the existing api's not be used here like iowrite64be/ioread64be/ or
>>> similar variant in "include/asm-generic/io.h"
>>
>> Oooh! I didn't realize that those exist. I'll change that and send a v2.
Yes, they are very new! It took me a while to get those patches in as it
is a bit more complicated than it seems.

> Actually, that doesn't work at the moment on x86-64:
> 
> include/asm-generic/io.h only defines these things if
> CONFIG_GENERIC_IOMAP isn't defined; and X86 unconditionally defines
> it. With CONFIG_GENERIC_IOMAP set, these functions are provided by
> include/asm-generic/iomap.h.

For CONFIG_GENERIC_IOMAP, lib/iomap.c provides _lo_hi and _hi_lo
implementations seeing the pio regions must be emulated with 32bit
operations and we have to define the order.

> The definitions for these are in lib/iomap.c, except that there are no
> definitions for ioread64be() and iowrite64be(); if you try to use
> them, you get linker errors.

Some platforms implement these but most do not. If I recall correctly
only powerpc does.

If you want to use 64 bit operations in a portable fashion, you should
include "linux/io-64-nonatomic-hi-lo.h" or
"linux/io-64-nonatomic-lo-hi.h", depending on weather you want the lower
bits or the higher bits to be written or read first in cases where an
atomic operation is not available.

Logan