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

[Takashi Iwai]

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
> >
> > We are using the internal sram available for sound, for DMA buffer
> > allocation, using the generic SRAM driver. From the above log, the
> > buffer address offset is not 8-byte aligned and as we are using the
> > SRAM driver which maps the memory as device memory we are getting an
> > unaligned fault. Is it incorrect to use the generic sram driver for
> > arm64 or am I missing something ?
> 
> When you give the ALSA framework a memory buffer, it is assumed that it
> is general-purpose memory, i.e., can be accessed with any alignment.
> 
> If your memory does not allow such accesses, you have to do all the
> accesses in your driver, i.e., do everything in the .copy/.silence
> callbacks.  (You also have to add constraints so that periods and buffer
> are correctly aligned.)

You can set runtime->min_align in the own hw_params callback so that
the PCM core handle it well, at least, for non-mmap mode.

But this feature has been rarely used (currently no driver sets it
explicitly), so there might be still some bugs there, too :)


Takashi