linux-kernel - Re: [PATCH 3/7] asm-generic/io.h: make ioread64 and iowrite64 universally available

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Date:   Thu, 22 Jun 2017 21:14:31 +0100
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-ntb@...glegroups.com, linux-alpha@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-crypto@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH 3/7] asm-generic/io.h: make ioread64 and iowrite64
 universally available

On Thu, 22 Jun 2017 10:48:13 -0600
Logan Gunthorpe <logang@...tatee.com> wrote:

> Currently, ioread64 and iowrite64 are only available io CONFIG_64BIT=y
> and CONFIG_GENERIC_IOMAP=n. Thus, seeing the functions are not
> universally available, it makes them unusable for driver developers.
> This leads to ugly hacks such as those at the top of
> 
> drivers/ntb/hw/intel/ntb_hw_intel.c
> 
> This patch adds fallback implementations for when CONFIG_64BIT and
> CONFIG_GENERIC_IOMAP are not set. These functions use two io32 based
> calls to complete the operation.
> 
> Note, we do not use the volatile keyword in these functions like the
> others in the same file. It is necessary to avoid a compiler warning
> on arm.

This is a really really bad idea as per the Alpha comment.

ioread64 and iowrite64 generate a single 64bit bus transaction. There is
hardware where mmio operations have side effects so simply using a pair
of 32bit operations blindly does not work (consider something as trivial
as reading a 64bit performance counter or incrementing pointer).

If a platform doesn't support 64bit I/O operations from the CPU then you
either need to use some kind of platform/architecture specific interface
if present or accept you don't have one.

It's not safe to split it. Possibly for some use cases you could add an

ioread64_maysplit()

but you cannot blindly break ioread64/write64() and expect it to
magically allow you to use drivers that depend upon it.

What btw is the actual ARM compiler warning ? Is the compiler also trying
to tell you it's a bad idea ?

Alan