From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933173AbcA0OQw (ORCPT ); Wed, 27 Jan 2016 09:16:52 -0500 Received: from devils.ext.ti.com ([198.47.26.153]:45154 "EHLO devils.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932766AbcA0OPO (ORCPT ); Wed, 27 Jan 2016 09:15:14 -0500 Subject: Re: [PATCH v3] ARM: edma: fix residue race for cyclic To: John Ogness , References: <87oacesxzy.fsf@linutronix.de> CC: , , , From: Peter Ujfalusi Message-ID: <56A8D0CA.9060307@ti.com> Date: Wed, 27 Jan 2016 16:14:34 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 MIME-Version: 1.0 In-Reply-To: <87oacesxzy.fsf@linutronix.de> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org One nitpick On 01/21/2016 06:22 PM, John Ogness wrote: > When retrieving the residue value for cyclic transfers, the > SRC/DST fields of the active PaRAM are read. However, the AM335x > Technical Reference Manual states: > > 11.3.3.6 Parameter Set Updates > > After the TR is read from the PaRAM (and is in the process > of being submitted to the EDMA3TC), the following fields are > updated as needed: ... SRC DST > > This means SRC/DST is incremented even though the DMA transfer > may not have started yet or is in progress. Thus if the reader > of the residue accesses the DMA buffer too quickly, the CPU is > misinformed about the data that has been successfully processed. > > The CCSTAT.ACTV register is a boolean that is set if any TR is > being processed by either the EMDA3CC or EDMA3TC. By polling > this register it is possible to ensure that the residue value > returned is valid for immediate processing. However, since the > DMA engine may be active, polling may never hit a moment where > no TR is being processed. To handle this, the SRC/DST is also > polled to see if it changes. And as a last resort, a max loop > count for the busy waiting exists to avoid an infinite loop. > > Signed-off-by: John Ogness > --- > v1-v2 changes > . rebased for next-20151016 > . added multiple exit conditions for busy wait loop > > v2-v3 changes > . rebased for next-20160121 > . reduced max loops from 10000 to 1000 > . loop countdown instead of count > . change debug print function and message > . fine-tune comments > > drivers/dma/edma.c | 40 +++++++++++++++++++++++++++++++++++++++- > 1 file changed, 39 insertions(+), 1 deletion(-) > > diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c > index d92d655..20cf643 100644 > --- a/drivers/dma/edma.c > +++ b/drivers/dma/edma.c > @@ -1680,9 +1680,22 @@ static void edma_issue_pending(struct dma_chan *chan) > spin_unlock_irqrestore(&echan->vchan.lock, flags); > } > > +#define EDMA_CCSTAT_ACTV (1 << 4) Can you move this to the define section of the file on top? > + > +/* > + * This limit exists to avoid a possible infinite loop when waiting for proof > + * that a particular transfer is completed. This limit can be hit if there > + * are large bursts to/from slow devices or the CPU is never able to catch > + * the DMA hardware idle. On an AM335x transfering 48 bytes from the UART > + * RX-FIFO, as many as 55 loops have been seen. > + */ > +#define EDMA_MAX_TR_WAIT_LOOPS 1000 > + > static u32 edma_residue(struct edma_desc *edesc) > { > bool dst = edesc->direction == DMA_DEV_TO_MEM; > + int loop_count = EDMA_MAX_TR_WAIT_LOOPS; > + struct edma_chan *echan = edesc->echan; > struct edma_pset *pset = edesc->pset; > dma_addr_t done, pos; > int i; > @@ -1691,7 +1704,32 @@ static u32 edma_residue(struct edma_desc *edesc) > * We always read the dst/src position from the first RamPar > * pset. That's the one which is active now. > */ > - pos = edma_get_position(edesc->echan->ecc, edesc->echan->slot[0], dst); > + pos = edma_get_position(echan->ecc, echan->slot[0], dst); > + > + /* > + * "pos" may represent a transfer request that is still being > + * processed by the EDMACC or EDMATC. We will busy wait until > + * any one of the situations occurs: > + * 1. the DMA hardware is idle > + * 2. a new transfer request is setup > + * 3. we hit the loop limit > + */ > + while (edma_read(echan->ecc, EDMA_CCSTAT) & EDMA_CCSTAT_ACTV) { > + /* check if a new transfer request is setup */ > + if (edma_get_position(echan->ecc, > + echan->slot[0], dst) != pos) { > + break; > + } > + > + if (!--loop_count) { > + dev_dbg_ratelimited(echan->vchan.chan.device->dev, > + "%s: timeout waiting for PaRAM update\n", > + __func__); > + break; > + } > + > + cpu_relax(); > + } > > /* > * Cyclic is simple. Just subtract pset[0].addr from pos. > -- Péter