dmaengine: at_xdmac: fix rare residue corruption

[Vinod Koul <vinod.koul@intel.com>]

On Thu, Feb 22, 2018 at 12:39:55PM +0100, Maxime Jayat wrote:
> Despite the efforts made to correctly read the NDA and CUBC registers,
> the order in which the registers are read could sometimes lead to an
> inconsistent state.
> 
> Re-using the timeline from the comments, this following timing of
> registers reads could lead to reading NDA with value "@desc2" and
> CUBC with value "MAX desc1":
> 
>  INITD --------                    ------------
>               |____________________|
>        _______________________  _______________
>  NDA       @desc2             \/   @desc3
>        _______________________/\_______________
>        __________  ___________  _______________
>  CUBC       0    \/ MAX desc1 \/  MAX desc2
>        __________/\___________/\_______________
>         |  |          |  |
> Events:(1)(2)        (3)(4)
> 
> (1) check_nda = @desc2
> (2) initd = 1
> (3) cur_ubc = MAX desc1
> (4) cur_nda = @desc2
> 
> This is allowed by the condition ((check_nda == cur_nda) && initd),
> despite cur_ubc and cur_nda being in the precise state we don't want.
> 
> This error leads to incorrect residue computation.
> 
> Fix it by inversing the order in which CUBC and INITD are read. This
> makes sure that NDA and CUBC are always read together either _before_
> INITD goes to 0 or _after_ it is back at 1.
> The case where NDA is read before INITD is at 0 and CUBC is read after
> INITD is back at 1 will be rejected by check_nda and cur_nda being
> different.

Applied, thanks