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

[David Miller <davem@davemloft.net>]

From: Roland Dreier <rdreier@cisco.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@cisco.com>

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