From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga06.intel.com ([134.134.136.31]:5384 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754524AbeCSHwE (ORCPT ); Mon, 19 Mar 2018 03:52:04 -0400 Date: Mon, 19 Mar 2018 13:26:09 +0530 From: Vinod Koul To: Ludovic Desroches Cc: Maxime Jayat , Nicolas Ferre , Dan Williams , dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] dmaengine: at_xdmac: fix rare residue corruption Message-ID: <20180319075608.GM15443@localhost> References: <20180222113955.4043-1-maxime.jayat@mobile-devices.fr> <20180301082516.GL22457@rfolt0960.corp.atmel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180301082516.GL22457@rfolt0960.corp.atmel.com> Sender: stable-owner@vger.kernel.org List-ID: 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 > > 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... > > 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