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SubjectRe: [PATCH] dmaengine: at_xdmac: fix rare residue corruption
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

--
~Vinod

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