Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute

From: Arnd Bergmann
Date: Mon Jun 20 2011 - 17:24:16 EST

On Monday 20 June 2011 22:55:59 Russell King - ARM Linux wrote:
> On Mon, Jun 20, 2011 at 10:26:37PM +0200, Arnd Bergmann wrote:
> > * We already need a compiler barrier in the non-_relaxed() versions of
> > the I/O accessors, which will force a reload of the base address
> > in a lot of cases, so the code is already suboptimal. Yes, we don't
> > have the barrier today without CONFIG_ARM_DMA_MEM_BUFFERABLE, but that
> > is a bug, because it lets the compiler move accesses to DMA buffers
> > around readl/writel.
> You're now being obtuse there. You don't need compiler barriers to
> guarantee order - that's what volatile does there.

A simple counterexample:

int f(volatile unsigned long *v)
unsigned long a[2], ret;
a[0] = 1; /* initialize our DMA buffer */
a[1] = 2;
*v = (unsigned long)a; /* pass the address to the device, start DMA */
ret = *v; /* flush DMA by reading from mmio */
return ret + a[1]; /* return accumulated status from readl and from modified
DMA buffer */

arm-linux-gnueabi-gcc -Wall -O2 test.c -S

Without a barrier, the stores into the DMA buffer before the start are
lost, as is the load from the modified DMA buffer:

sub sp, sp, #8
add r3, sp, #0
str r3, [r0, #0]
ldr r0, [r0, #0]
adds r0, r0, #2
add sp, sp, #8
bx lr

Adding a memory clobber to the volatile dereference turns this into the
expected output:

sub sp, sp, #8
movs r3, #2
movs r2, #1
stmia sp, {r2, r3}
add r3, sp, #0
str r3, [r0, #0]
ldr r0, [r0, #0]
ldr r3, [sp, #4]
adds r0, r0, r3
add sp, sp, #8
bx lr

Now, the dma buffer is written before the volatile access, and read out
again afterwards.

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