linux-kernel - Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

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Message-Id: <20090428.221228.217954247.davem@davemloft.net>
Date:	Tue, 28 Apr 2009 22:12:28 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rdreier@...co.com
Cc:	hpa@...or.com, mingo@...e.hu, tglx@...utronix.de,
	h.mitake@...il.com, rpjday@...shcourse.ca,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

From: Roland Dreier <rdreier@...co.com>
Date: Tue, 28 Apr 2009 12:05:10 -0700

> As discussed in <http://lkml.org/lkml/2009/4/19/164> and follow-ups,
> readq()/writeq() for 32-bit x86 are implemented as two readl()/writel()
> operations.  This is not atomic (in the sense that another MMIO
> operation from another CPU or thread can be done in the middle of the
> two read/writes), and may not access the two halves of the register in
> the correct order to work with hardware.
> 
> Rather than silently providing a 32-bit fallback that leaves a
> possibility for strange driver bugs, it's better to provide readq()
> and writeq() only for 64-bit architectures, and have a compile failure
> on 32-bit architectures that forces driver authors to think about what
> the correct solution is.
> 
> This essentially reverts 2c5643b1 ("x86: provide readq()/writeq() on
> 32-bit too") and follow-on commits.  If in the future someone wants to
> provide a generic solution for all 32-bit architectures, that's great,
> but there's not much point in providing (arguably broken)
> implementations for only one architecture, since any portable driver
> will have to implement fallbacks for other architectures anyway.
> 
> Signed-off-by: Roland Dreier <rolandd@...co.com>

Acked-by: David S. Miller <davem@...emloft.net>
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