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

[Ludovic Desroches <ludovic.desroches@microchip.com>]

On Mon, Mar 19, 2018 at 01:26:09PM +0530, Vinod Koul wrote:
> On Thu, Mar 01, 2018 at 09:25:16AM +0100, Ludovic Desroches wrote:
> > Hi Maxime,
> > 
> > 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.
> > > 
> > > Fixes: 53398f488821 ("dmaengine: at_xdmac: fix residue corruption")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Maxime Jayat <maxime.jayat@mobile-devices.fr>
> > 
> > Nice work! I agree with the change you propose.
> > 
> > I am disappointed we didn't spot this case so I would like to double-check with
> > the hardware guy there is no issue with the sequence you propose. That's
> > why I am waiting a bit before giving my ack.
> 
> any update on that? This has been pending for a while...

Unfortunately not. As I am pretty confident in Maxime patch:
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>

> 
> > 
> > Regards
> > 
> > Ludovic
> > 
> > > ---
> > > Hi,
> > > 
> > > I had a bug where the serial ports on the Atmel SAMA5D2 were sometimes
> > > returning the same data twice, for up to 4096 bytes.
> > > 
> > > After investigation, I noticed that the ring buffer used in
> > > atmel_serial (in rx dma mode) had sometimes a incorrect "head" value,
> > > which made the ring buffer do a complete extraneous loop of data
> > > pushed to the tty layer.
> > > 
> > > I tracked it down to the residue of the dma being wrong, and after
> > > more head scratching, I found this bug in the reading of the
> > > registers.
> > > 
> > > Before fixing this, I was able to reproduce the bug reliably in a few
> > > minutes. With this patch applied, the bug did not reappear after
> > > several hours in testing.
> > > 
> > > 
> > >  drivers/dma/at_xdmac.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
> > > index c00e3923d7d8..94236ec9d410 100644
> > > --- a/drivers/dma/at_xdmac.c
> > > +++ b/drivers/dma/at_xdmac.c
> > > @@ -1471,10 +1471,10 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
> > >  	for (retry = 0; retry < AT_XDMAC_RESIDUE_MAX_RETRIES; retry++) {
> > >  		check_nda = at_xdmac_chan_read(atchan, AT_XDMAC_CNDA) & 0xfffffffc;
> > >  		rmb();
> > > -		initd = !!(at_xdmac_chan_read(atchan, AT_XDMAC_CC) & AT_XDMAC_CC_INITD);
> > > -		rmb();
> > >  		cur_ubc = at_xdmac_chan_read(atchan, AT_XDMAC_CUBC);
> > >  		rmb();
> > > +		initd = !!(at_xdmac_chan_read(atchan, AT_XDMAC_CC) & AT_XDMAC_CC_INITD);
> > > +		rmb();
> > >  		cur_nda = at_xdmac_chan_read(atchan, AT_XDMAC_CNDA) & 0xfffffffc;
> > >  		rmb();
> > >  
> > > -- 
> > > 2.14.1
> > > 
> 
> -- 
> ~Vinod