Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751921AbaJ0Py7 (ORCPT ); Mon, 27 Oct 2014 11:54:59 -0400 Received: from foss-mx-na.foss.arm.com ([217.140.108.86]:49133 "EHLO foss-mx-na.foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751060AbaJ0Py6 (ORCPT ); Mon, 27 Oct 2014 11:54:58 -0400 Date: Mon, 27 Oct 2014 15:54:45 +0000 From: Will Deacon To: Catalin Marinas Cc: Mathieu Poirier , "thomas.petazzoni@free-electrons.com" , "linux@arm.linux.org.uk" , "stefano.stabellini@eu.citrix.com" , Liviu Dudau , "linux-kernel@vger.kernel.org" , "ezequiel.garcia@free-electrons.com" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH] ARM: supplementing IO accessors with 64 bit capability Message-ID: <20141027155444.GW8768@arm.com> References: <1413993983-17310-1-git-send-email-mathieu.poirier@linaro.org> <20141022164454.GI15370@e104818-lin.cambridge.arm.com> <20141024092832.GD20534@e104818-lin.cambridge.arm.com> <20141024161634.GG20534@e104818-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141024161634.GG20534@e104818-lin.cambridge.arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 24, 2014 at 05:16:34PM +0100, Catalin Marinas wrote: > On Fri, Oct 24, 2014 at 04:05:13PM +0100, Mathieu Poirier wrote: > > On 24 October 2014 03:28, Catalin Marinas wrote: > > > On Wed, Oct 22, 2014 at 08:10:27PM +0100, Mathieu Poirier wrote: > > >> On 22 October 2014 18:44, Catalin Marinas wrote: > > >> > On Wed, Oct 22, 2014 at 05:06:23PM +0100, mathieu.poirier@linaro.org wrote: > > >> >> +static inline void __raw_writeq(u64 val, volatile void __iomem *addr) > > >> >> +{ > > >> >> + asm volatile("strd %1, %0" > > >> >> + : "+Qo" (*(volatile u64 __force *)addr) > > >> >> + : "r" (val)); > > >> >> +} > > >> >> + > > >> >> +static inline u64 __raw_readq(const volatile void __iomem *addr) > > >> >> +{ > > >> >> + u64 val; > > >> >> + asm volatile("ldrd %1, %0" > > >> >> + : "+Qo" (*(volatile u64 __force *)addr), > > >> >> + "=r" (val)); > > >> >> + return val; > > >> >> +} > > >> >> +#endif > > >> > > > >> > I'm curious why you need these. Do you have a device that needs a 64-bit > > >> > single access or you are trying to read two consecutive registers? > > >> > > >> The fundamental data size of Coresight STM32 for ARMv7 is > > >> implementation defined and can be 32 or 64bit. As such stimulus ports > > >> can support transaction sizes of up to 64 bit. > > > > > > The STM programmer's model spec recommends something else (though I find > > > the "3.6 Data sizes" chapter a bit confusing): > > > > > > To ensure that code is portable between processor micro-architectures > > > and system implementations, ARM recommends that only the native data > > > size of the machine is used, and smaller sizes. For the 32-bit ARMv7 > > > architecture, only 8, 16, and 32-bit transfers are recommended. For an > > > ARMv8 processor that supports the AArch64 Execution state, it is > > > recommended that the fundamental data size of 64-bits is implemented. > > > > > > Which means that you should not use readq/writeq on a 32-bit system. > > > > Not quite. ARM documentation IHI0054B (ARM System Trace Macrocell: > > Programmers' Model Architecture Specification) stipulate that "For > > systems with an ARMv7 processor, ARM recommends configuration 1 or > > configuration 2.", where configuration 2 has a fundamental size of 64 > > bit. > > As I said, it's confusing. Anyway, you can go ahead and add the > readq/writeq for ARMv6 and later, though it won't be guaranteed to have > a 64-bit access, it depends on the bus. I'm really not comfortable with this... we don't make any guarantees for 32-bit CPUs that a double-word access will be single-copy atomic for MMIO. That means it could be subjected to things like reordering and merging, which I think means that it depends on both the bus *and* the endpoint as to whether or not this will work. Worse still, the endpoint could decide to return a SLVERR, which would appear as an external abort. Is it not possible to use 32-bit MMIO accesses for this driver? Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/