linux-kernel - Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE

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Message-Id: <20090421.172547.85702227.davem@davemloft.net>
Date:	Tue, 21 Apr 2009 17:25:47 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rdreier@...co.com
Cc:	hpa@...or.com, h.mitake@...il.com, mingo@...e.hu,
	tglx@...utronix.de, rpjday@...shcourse.ca,
	linux-kernel@...r.kernel.org
Subject: Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars

From: Roland Dreier <rdreier@...co.com>
Date: Tue, 21 Apr 2009 10:07:50 -0700

> This only makes sense if we define a 32-bit fallback for
> readq()/writeq() for all 32-bit architectures -- in fact it would be
> good to do it in asm-generic so that there can be a single
> implementation that guarantees that non-atomic versions always do, say,
> low 32 bits then high 32 bits.  (So eg niu can use the generic version)
> And then drivers like drivers/infiniband/hw/mthca can be switched to
> "#ifndef writeq_atomic <...hardware specific fallback...>"

I think if you want to do this right you have to provide two versions
of the 32-bit implementations, when the cpu cannot generate full
64-bit transactions.  Especially for readq().

Some devices clear the status bits of a 64-bit register when read, so
it might matter deeply whether the top-half or the bottom-half 32-bits
are read first.

The following are ugly names, but something like "readq_hifirst()"
and "readq_lofirst()" and they just get defined to play "readq()"
in situations where a full 64-bit transaction can be generated by
the cpu.

The driver author has to figure out which is appropriate.

And I'm pretty sure similar high-first/low-first issues can exist
for writeq() as well.
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