Re: [PATCH] arm: Remove IO memcpy for Big-Endian

[Julian Vetter]

On 03.12.24 11:08, Arnd Bergmann wrote:
> On Tue, Dec 3, 2024, at 09:38, Julian Vetter wrote:
> > From: Julian Vetter <julian@xxxxxxxxxxxxxxxx>
> >
> > Recently a new IO memcpy was added in libs/iomem_copy.c. So, remove the
> > byte-wise IO memcpy operations used in ARM big endian builds and fall
> > back to the new generic implementation. It will be slightly faster,
> > because it uses machine word accesses if the memory is aligned and falls
> > back to byte-wise accesses if its not.
> >
> > Signed-off-by: Julian Vetter <julian@xxxxxxxxxxxxxxxx>
> > ---
> >   arch/arm/include/asm/io.h | 11 ----------
> >   arch/arm/kernel/io.c      | 46 ---------------------------------------
> >   2 files changed, 57 deletions(-)

Hello Arnd,
first, sorry, that I messed up my sender email while sending the patch.

> I'm not sure if this is safe on all platforms. Big-endian arm
> is extremely rare in practice, and in comes in multiple variants
> that behave slightly differently:
>
> - On modern ARMv7 the byte-invariant big-endian "BE8" mode
>    generally well-behaved and works as one would expect it to.
>
> - There is one ARMv5 "BE32" based platform, the ixp4xx, which
>    works differently, and this in turn allows multiple configurations
>    for its buses where a byte-swap is performed in the PCI
>    controller.
>
> When the little-endian I/O string operations got optimized to
> calling the word-based helpers in commit 7ddfe625cbc1 ("ARM:
> optimize memset_io()/memcpy_fromio()/memcpy_toio()"), Russell
> intentionally left the big-endian versions alone, which I think
> was done for the case of PCI on ixp4xx, but could have been
> out of general caution.

Wow, thank you for the explanation!

> Before we apply your patch, I think the minimum would be to
> have Linus Walleij try it out on an an ixp4xx with a driver
> that uses these functions. Maybe Russell remembers the exact
> constraints that led to using byte access for big-endian
> mmio string operations, and whether the new lib/iomem_copy.c
> version causes problems.
>
> I also looked at the little-endian arm32 version, and
> it seems that here the generic code would work fine, but
> the custom variant is likely much faster when both the
> source and destination buffers are aligned, as it can
> do larger MMIO transactions using ldm/stm instructions,
> though the generic version would be a bit better if the
> in-memory buffer is unaligned.
> We could get the best of both by implementing optimized
> arm32 versions __iowrite32_copy()/__ioread32_copy and
> using those in the generic memcpy_{from,to}io.

ok, then I will wait for now, to see if it works with the current 
version. If not, I will have a look at using the 
__iowrite32_copy/__ioread32_copy functions.

>         Arnd