From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Walker Subject: Re: [PATCH v2 2/2] msm: DMAEngine: Add DMAEngine driver based on old MSM DMA APIs Date: Sat, 07 Jan 2012 16:13:56 -0800 Message-ID: <1325981636.16037.6.camel@m0nster> References: <1325854052-21402-1-git-send-email-kumarrav@codeaurora.org> <1325854052-21402-3-git-send-email-kumarrav@codeaurora.org> <1325901569.10425.14.camel@m0nster> <20120107185443.GA19216@codeaurora.org> <20120107192112.GB13181@n2100.arm.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: Received: from fifo99.com ([67.223.236.141]:51270 "EHLO fifo99.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752067Ab2AHAOG (ORCPT ); Sat, 7 Jan 2012 19:14:06 -0500 In-Reply-To: <20120107192112.GB13181@n2100.arm.linux.org.uk> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Russell King - ARM Linux Cc: David Brown , Ravi Kumar V , vinod.koul@intel.com, dan.j.williams@intel.com, arnd@arndb.de, linux-arch@vger.kernel.org, bryanh@codeaurora.org, tsoni@qualcomm.com, johlstei@qualcomm.com, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Nicolas Ferre On Sat, 2012-01-07 at 19:21 +0000, Russell King - ARM Linux wrote: > On Sat, Jan 07, 2012 at 10:54:43AM -0800, David Brown wrote: > > On Fri, Jan 06, 2012 at 05:59:29PM -0800, Daniel Walker wrote: > > > > > > diff --git a/drivers/dma/msm-dma.c b/drivers/dma/msm-dma.c > > > > new file mode 100644 > > > > index 0000000..51d9a2b > > > > --- /dev/null > > > > +++ b/drivers/dma/msm-dma.c > > > > ... > > > > +static void msm_chan_desc_cleanup(struct msm_dma_chan *chan) > > > > +{ > > > > + struct msm_dma_desc_sw *desc, *_desc; > > > > + unsigned long flags; > > > > + > > > > + dev_dbg(chan->dev, "Cleaning completed descriptor of channel %d\n", > > > > + chan->chan_id); > > > > + spin_lock_irqsave(&chan->lock, flags); > > > > + > > > > + list_for_each_entry_safe(desc, _desc, &chan->active_list, node) { > > > > + dma_async_tx_callback callback; > > > > + void *callback_param; > > > > + > > > > + if (msm_dma_desc_status(chan, desc) == DMA_IN_PROGRESS) > > > > + break; > > > > + > > > > + /* Remove from the list of running transactions */ > > > > + list_del(&desc->node); > > > > + > > > > + /* Run the link descriptor callback function */ > > > > + callback = desc->async_tx.callback; > > > > + callback_param = desc->async_tx.callback_param; > > > > + if (callback) { > > > > + spin_unlock_irqrestore(&chan->lock, flags); > > > > + callback(callback_param); > > > > > > Are you sure unlocking here is safe? at_hdmac.c holds the lock the > > > entire time, and fsldma.c deletes the entire list, then runs the > > > operations. > > > > Good catch. > > > > According to a comment in at_hdmac.c, it is safe to hold the lock > > while calling the callbacks, so that's probably the easiest solution. > > I suspect that you've got something in another driver expecting the > > lock to be released, and that might have to be changed. > > It is _not_ safe to hold the lock while calling callbacks. > > Please refer to the DMA engine documentation, found here: > > Documentation/dmaengine.txt > > section 3: > > Note: > Although the async_tx API specifies that completion callback > routines cannot submit any new operations, this is not the > case for slave/cyclic DMA. > > For slave DMA, the subsequent transaction may not be available > for submission prior to callback function being invoked, so > slave DMA callbacks are permitted to prepare and submit a new > transaction. > > For cyclic DMA, a callback function may wish to terminate the > DMA via dmaengine_terminate_all(). > > * Therefore, it is important that DMA engine drivers drop any > * locks before calling the callback function which may cause a > * deadlock. Here's the comment from at_hdmac.c . /* * The API requires that no submissions are done from a * callback, so we don't need to drop the lock here */ if (callback) callback(param); I don't know much about the DMA engine, but maybe there is some special case here that makes this ok.. (CC'ed Nicolas Ferre) Daniel