Re: Unaligned access in kernel on ARMv6+ (Re: CVS commit: src/sys/dev/usb)

[Nick Hudson <nick.hudson%gmx.co.uk@localhost>]

On 07/01/2019 10:22, Rin Okuyama wrote:
[snip]

> This kind of problems cannot be handled in software unless we
>
>      (1) use cached memory (for which unaligned access is allowed), or
>      (2) forbid compiler to generate unaligned access.
>
> This patch
>
>      http://www.netbsd.org/~rin/armv6_cached_dma_memory_20180107.patch
>
> adds ARMV6_CACHED_DMA_MEMORY option. If enabled, DMA memory is forcibly
> mapped cacheable on ARMv6+ [option (1) above]. This allows us unaligned
> access to DMA buffer, however, as Nick and others pointed out, breaks
> drivers that do not invalidate cache appropriately. If this option is
> disabled, -mno-unaligned-access is added to CFLAGS [option (2) above].
>
> I've tested both on my RPI3B+ (earmv7hf). Kernel of (1) works more than
> 12 hours without panic. However, vchiq(4) fails to initialize, and mue(4)
> receives strange packets of zero-length (two times in 12 hours). Both
> smell like driver bugs. Kernel of (2) works without problems as far as
> I can see.

Does it work on rpi1? I forget if ARM11_COMPAT_MMU behaves differently here.

> I also carried out simple benchmarks of building lang/perl5 of pkgsrc.
> The working directory is USB SSD, TMPDIR is tmpfs, and terminal is ssh.
> The difference is negligible: (1) 25:36.53 and (2) 25:34.81.

Using cached memory for DMA is slower?

> We should use cached memory for DMA in the future. However, it may break
> more drivers than I observed on RPI. Therefore, I would like to propose
> a compromise plan:
>
> (a) Before branching netbsd-9, disable ARMV6_CACHED_MEMORY, and use
>      -mno-unaligned-access.
>
> (b) After branching netbsd-9, enable ARMV6_CACHED_MEMORY, and stop using
>      -mno-unaligned-access.
>
> (c) After debugging drivers, use cached memory for DMA unconditionally
>      on ARMv6+ and remove ARMV6_CACHED_DMA_MEMORY option.
>
> Thoughts? Nick, does this look reasonable for you?

OK.

Let's backport all related fixes and plan to remove 
ARMV6_CACHED_DMA_MEMORY everywhere. (pending the rpi result)	

Thanks,
Nick