[alsa-devel] Question on arm64 unaligned faults during playback

[Catalin Marinas]

On Mon, Dec 08, 2014 at 11:15:52AM +0000, Lars-Peter Clausen wrote:
> Added ARM64 folks to Cc.

Thanks.

> On 12/08/2014 11:41 AM, Takashi Iwai wrote:
> > At Mon, 08 Dec 2014 09:49:37 +0100,
> > Clemens Ladisch wrote:
> >>
> >> Abhilash Kesavan wrote:
> >>> I am working on a 64-bit ARM SoC (Samsung's Exynos7) and have observed
> >>> unaligned faults while testing certain sound streams with aplay.
> >>>
> >>> [   24.535661] snd_pcm_lib_write_transfer:sound/core/pcm_lib.c hwbuf is ffffff8000085624, runtime->dma_area is ffffff8000080000, hwoff is 5513, frames_to_bytes is 22052, frames is 5513
> >>> [   24.551244] Unhandled fault: alignment fault (0x96000061) at 0xffffff8000085624
> >>> [   24.579944] PC is at __copy_from_user+0x14/0x60
> >>> [   24.584450] LR is at snd_pcm_lib_write_transfer+0xe4/0x104
> >>> [   24.922054] Call trace:
> >>> [   24.924488] [<ffffffc0002d4784>] __copy_from_user+0x14/0x60
> >>> [   24.930040] [<ffffffc0004b7574>] snd_pcm_lib_write1+0x1fc/0x384

I can see copy_from_user() here, I assume the source is a user space
buffer and the destination is the ioremap'ed SRAM (which causes the
unaligned fault). Such function is not supposed copy into device memory
(its prototype does not specify __iomem for the destination pointer).

If you really want Device memory for the SRAM, I think we need some
other API. memcpy_toio() could copy into such buffer but the source must
be kernel. Maybe something like copy_from_user_toio()?

Otherwise, if you can remap the buffer as Normal (non-cacheable) memory,
you could use something like vmap() with pgprot_writecombine for the
mapping. Even though ioremap_wc() returns (currently) Normal
non-cacheable memory, such pointer (__iomem) should only be used with
specific IO accessors (though I can see this assumption broken in a few
places in the kernel).

-- 
Catalin