Hello. 

Sorry for my unfamiliarity with the kernel community. I'm afraid I'll
be getting the procedure wrong...

I was using linux-libre-3.19 (implies no working sdma) with a
wandboard quad (Freescale imx6q). Spidf output worked fine.

When I upgraded to linux-libre-4.7 spdif output was supressed without
any error (precisely, with the same errors about sdma that 3.19 gave).

I saw someone else reporting the same elsewhere with linux-4.4
https://forum.digikey.com/thread/34240
(but I don't have a login there) 

This patch fixes it for me and sound works again on spdif. 

But I don't know if it can break (or fix?) something for some other
boards or kernels or cases... I hardly know what I'm doing.

The commits that might have caused the problem for me might be 

commit 833f2cbf7091099baee28136dc68678e974c0ac5
Author: Shengjiu Wang <shengjiu.wang@freescale.com>
Date:   Sat Oct 10 18:15:07 2015 +0800

    ARM: dts: imx6: change the core clock of spdif
    
    The correct core clock of spdif is SPDIF_GCLK, which is added to
    clock tree. So the dts also need to be updated.
    
    Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
    Signed-off-by: Shawn Guo <shawnguo@kernel.org>

(the commit changed more clocks than SPDIF_GCLK) 

It might have to do with vendor commit 

http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git/commit/arch/arm/boot/dts/imx6qdl.dtsi?h=imx_4.1.15_1.0.0_ga&id=dd7ebdf5bf0458e08f0be62fd49bc6f1ca9b2f25

The issue is fixed for me with this patch but I'm not sure what's the
best way to help fix any issue someone else may have or what other
info or test you might need. Any guidance welcome. 

Thank you for linux.