linux-kernel - Re: [PATCH 22/24] scsi: eesox: use _

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Message-ID: <20120917220334.GU12245@n2100.arm.linux.org.uk>
Date:	Mon, 17 Sep 2012 23:03:34 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Will Deacon <will.deacon@....com>,
	Nicolas Pitre <nico@...aro.org>,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH 22/24] scsi: eesox: use __iomem pointers for MMIO

On Sat, Sep 15, 2012 at 10:30:43AM +0000, Arnd Bergmann wrote:
> On Saturday 15 September 2012, Russell King - ARM Linux wrote:
> > On Sat, Sep 15, 2012 at 08:00:35AM +0000, Arnd Bergmann wrote:
> > > On Friday 14 September 2012, Russell King - ARM Linux wrote:
> > > > On Fri, Sep 14, 2012 at 11:34:50PM +0200, Arnd Bergmann wrote:
> > > > > ARM is moving to stricter checks on readl/write functions,
> > > > > so we need to use the correct types everywhere.
> > > > 
> > > > There's nothing wrong with const iomem pointers.  If you think
> > > > otherwise, patch x86 not to use const in its accessor implementation
> > > > and watch the reaction:
> > > > 
> > > > #define build_mmio_read(name, size, type, reg, barrier) \
> > > > static inline type name(const volatile void __iomem *addr) \
> > > > { type ret; asm volatile("mov" size " %1,%0":reg (ret) \
> > > > :"m" (*(volatile type __force *)addr) barrier); return ret; }
> > > > 
> > > > build_mmio_read(readb, "b", unsigned char, "=q", :"memory")
> > > > build_mmio_read(readw, "w", unsigned short, "=r", :"memory")
> > > > build_mmio_read(readl, "l", unsigned int, "=r", :"memory")
> > > 
> > > Ok, fair enough. Can you fold the patch below into 
> > > "ARM: 7500/1: io: avoid writeback addressing modes for __raw_
> > > accessors", or apply on top then?
> > 
> > No - const is not appropriate for the write accessors.  Again, this puts
> > us at odds with x86:
> > 
> > #define build_mmio_write(name, size, type, reg, barrier) \
> > static inline void name(type val, volatile void __iomem *addr) \
> > { asm volatile("mov" size " %0,%1": :reg (val), \
> > "m" (*(volatile type __force *)addr) barrier); }
> > 
> > build_mmio_write(writeb, "b", unsigned char, "q", :"memory")
> > build_mmio_write(writew, "w", unsigned short, "r", :"memory")
> > build_mmio_write(writel, "l", unsigned int, "r", :"memory")
> > 
> > So, readl etc are all const volatile void __iomem *, but writel etc are
> > all volatile void __iomem *.
> > 
> > How they're defined on ARM after 7500/1 copies how they're defined on
> > x86.
> 
> Well, you have to make up your mind what you want. Right now, we get these
> warnings in rpc_defconfig:
> 
>   Generating include/generated/mach-types.h
> /home/arnd/linux-arm/drivers/net/ethernet/seeq/ether3.c: In function 'ether3_outb':
> /home/arnd/linux-arm/drivers/net/ethernet/seeq/ether3.c:104:2: error: passing argument 2 of '__raw_writeb' discards 'const' qualifier from pointer target type [-Werror]
> /home/arnd/linux-arm/arch/arm/include/asm/io.h:81:91: note: expected 'volatile void *' but argument is of type 'const void *'
> /home/arnd/linux-arm/drivers/scsi/arm/eesox.c: In function 'eesoxscsi_buffer_out':
> /home/arnd/linux-arm/drivers/scsi/arm/eesox.c:310:4: error: passing argument 2 of '__raw_writel' discards 'const' qualifier from pointer target type [-Werror]
> /home/arnd/linux-arm/arch/arm/include/asm/io.h:88:91: note: expected 'volatile void *' but argument is of type 'const void *'
> /home/arnd/linux-arm/drivers/scsi/arm/eesox.c:324:4: error: passing argument 2 of '__raw_writel' discards 'const' qualifier from pointer target type [-Werror]
> /home/arnd/linux-arm/arch/arm/include/asm/io.h:88:91: note: expected 'volatile void *' but argument is of type 'const void *'
> /home/arnd/linux-arm/drivers/scsi/arm/eesox.c:325:4: error: passing argument 2 of '__raw_writel' discards 'const' qualifier from pointer target type [-Werror]
> /home/arnd/linux-arm/arch/arm/include/asm/io.h:88:91: note: expected 'volatile void *' but argument is of type 'const void *'
> 
> Either we allow drivers to write to const __iomem pointers or we don't. I
> don't care which way we do it, but just saying both patches are wrong is
> not very helpful.

In both of my replies, I've said "as x86 does".  We need to follow
x86's behaviour here, which is as I've quoted - it's not a matter
that "I need to make up my mind" - my mind is already well made up.
That is, we need to follow x86 here.

That is, const volatile void __iomem * for reads, and volatile void
__iomem * for writes.
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