linux-kernel - Re: [PATCH] Fixes for dw_dmac and atmel-mci for AP700x

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Date:	Tue, 21 Aug 2012 22:24:11 +0800
From:	Hein Tibosch <hein_tibosch@...oo.es>
To:	Arnd Bergmann <arnd.bergmann@...aro.org>
CC:	Viresh Kumar <viresh.kumar@...aro.org>,
	Hans-Christian Egtvedt <hans-christian.egtvedt@...el.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Havard Skinnemoen <havard@...nnemoen.net>,
	"ludovic.desroches" <ludovic.desroches@...el.com>,
	linux-kernel@...r.kernel.org, spear-devel <spear-devel@...t.st.com>
Subject: Re: [PATCH] Fixes for dw_dmac and atmel-mci for AP700x

On 8/21/2012 5:05 PM, Arnd Bergmann wrote:
> It should be easy to tell from the object code whether this happened
> or not. If it did, then we can investigate why gcc did that, otherwise
> something else caused the strange byte swap.
>
> The safe way to define the readl() function in asm/io.h is to
> use an inline assembly that prevents the access from getting split,
> but avr32 just uses a pointer dereference here.
>
> I think I just found the answer elsewhere in
> arch/avr32/mach-at32ap/include/mach/io.h, which defines
>
> # define __mem_ioswabl(a, x)    swahb32(x)
>
> and that apparently does the halfword swap when CONFIG_AP700X_16_BIT_SMC
> is set. This explains why Havard said it's wrong to use readl on
> internal deviceson avr32, but unfortunately that rule conflicts with how
> we define the accessors on ARM.

I already thought the 16-bit swap might be related to the 16-bit SMC
configuration. SMC will fetch each u32 word in 2 different 16-bit banks
of SDRAM.

In a meanwhile I tried dw_dmac using iowrite32be/ioread32be, and it worked
equally well! Which isn't surprising because for AVR32 they were defined as:

#define iowrite32be(v,p)    __raw_writel(v, p)
#define ioread32be(p)        ((unsigned int)__raw_readl(p))

The 'relaxed' versions won't work because:

#define readl_relaxed            readl

The object code confirms what was expected:

    u32 val = ioread32be (((unsigned*)0x100)); /* Same as __raw_readl */
    ld.w  r8,pc[256]

    iowrite32be (val, ((unsigned*)0x104)); /* same as __raw_writel */
    st.w  pc[260],r8

    val = readl (((unsigned*)0x108)); /* __raw_readl + swahb32 */
    ld.w  r8,pc[264]
    lsl   r9,r8,0x10  /* 16-bit swap */
    or    r9,r9,r8>>0x10

    writel (val, ((unsigned*)0x10C)); /* swahb32 + __raw_writel */
    lsl   r8,r9,0x10  /* 16-bit swap */
    or    r8,r8,r9>>0x10
    st.w  pc[268],r8


Hein
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