Subject: Re: [PATCH v3 3/4] fsl-dma: change release process of dma descriptor for supporting async_tx

From: "Ira W. Snyder" <[email-masked]>
Date: Tue, 17 Jul 2012 09:16:55 -0700

On Tue, Jul 17, 2012 at 07:06:33AM +0000, Liu Qiang-B32616 wrote:
> > -----Original Message-----
> > From: Ira W. Snyder [mailto:[email-masked]]
> > Sent: Tuesday, July 17, 2012 4:01 AM
> > To: Liu Qiang-B32616
> > Cc: [email-masked]; [email-masked]; Phillips
> > Kim-R1AAHA; [email-masked]; [email-masked]; Dan
> > Williams; Vinod Koul; Li Yang-R58472
> > Subject: Re: [PATCH v3 3/4] fsl-dma: change release process of dma
> > descriptor for supporting async_tx
> > 
> > On Mon, Jul 16, 2012 at 12:08:29PM +0800, Qiang Liu wrote:
> > > Fix the potential risk when enable config NET_DMA and ASYNC_TX.
> > > Async_tx is lack of support in current release process of dma
> > > descriptor, all descriptors will be released whatever is acked or
> > > no-acked by async_tx, so there is a potential race condition when dma
> > > engine is uesd by others clients (e.g. when enable NET_DMA to offload
> > TCP).
> > >
> > > In our case, a race condition which is raised when use both of talitos
> > > and dmaengine to offload xor is because napi scheduler will sync all
> > > pending requests in dma channels, it affects the process of raid
> > > operations due to ack_tx is not checked in fsl dma. The no-acked
> > > descriptor is freed which is submitted just now, as a dependent tx,
> > > this freed descriptor trigger
> > > BUG_ON(async_tx_test_ack(depend_tx)) in async_tx_submit().
> > >
> > > Cc: Dan Williams <[email-masked]>
> > > Cc: Vinod Koul <[email-masked]>
> > > Cc: Li Yang <[email-masked]>
> > > Cc: Ira W. Snyder <[email-masked]>
> > > Signed-off-by: Qiang Liu <[email-masked]>
> > > ---
> > >  drivers/dma/fsldma.c |  378 +++++++++++++++++++++++++++++-------------
> > -------
> > >  drivers/dma/fsldma.h |    1 +
> > >  2 files changed, 225 insertions(+), 154 deletions(-)
> > >
> > > diff --git a/drivers/dma/fsldma.c b/drivers/dma/fsldma.c index
> > > 4f2f212..4ee1b8f 100644
> > > --- a/drivers/dma/fsldma.c
> > > +++ b/drivers/dma/fsldma.c
> > > @@ -400,6 +400,217 @@ out_splice:
> > >  	list_splice_tail_init(&desc->tx_list, &chan->ld_pending);  }
> > >
> > > +/**
> > > + * fsl_chan_xfer_ld_queue - transfer any pending transactions
> > > + * @chan : Freescale DMA channel
> > > + *
> > > + * HARDWARE STATE: idle
> > > + * LOCKING: must hold chan->desc_lock  */ static void
> > > +fsl_chan_xfer_ld_queue(struct fsldma_chan *chan) {
> > > +	struct fsl_desc_sw *desc;
> > > +
> > > +	/*
> > > +	 * If the list of pending descriptors is empty, then we
> > > +	 * don't need to do any work at all
> > > +	 */
> > > +	if (list_empty(&chan->ld_pending)) {
> > > +		chan_dbg(chan, "no pending LDs\n");
> > > +		return;
> > > +	}
> > > +
> > > +	/*
> > > +	 * The DMA controller is not idle, which means that the interrupt
> > > +	 * handler will start any queued transactions when it runs after
> > > +	 * this transaction finishes
> > > +	 */
> > > +	if (!chan->idle) {
> > > +		chan_dbg(chan, "DMA controller still busy\n");
> > > +		return;
> > > +	}
> > > +
> > > +	/*
> > > +	 * If there are some link descriptors which have not been
> > > +	 * transferred, we need to start the controller
> > > +	 */
> > > +
> > > +	/*
> > > +	 * Move all elements from the queue of pending transactions
> > > +	 * onto the list of running transactions
> > > +	 */
> > > +	chan_dbg(chan, "idle, starting controller\n");
> > > +	desc = list_first_entry(&chan->ld_pending, struct fsl_desc_sw,
> > node);
> > > +	list_splice_tail_init(&chan->ld_pending, &chan->ld_running);
> > > +
> > > +	/*
> > > +	 * The 85xx DMA controller doesn't clear the channel start bit
> > > +	 * automatically at the end of a transfer. Therefore we must clear
> > > +	 * it in software before starting the transfer.
> > > +	 */
> > > +	if ((chan->feature & FSL_DMA_IP_MASK) == FSL_DMA_IP_85XX) {
> > > +		u32 mode;
> > > +
> > > +		mode = DMA_IN(chan, &chan->regs->mr, 32);
> > > +		mode &= ~FSL_DMA_MR_CS;
> > > +		DMA_OUT(chan, &chan->regs->mr, mode, 32);
> > > +	}
> > > +
> > > +	/*
> > > +	 * Program the descriptor's address into the DMA controller,
> > > +	 * then start the DMA transaction
> > > +	 */
> > > +	set_cdar(chan, desc->async_tx.phys);
> > > +	get_cdar(chan);
> > > +
> > > +	dma_start(chan);
> > > +	chan->idle = false;
> > > +}
> > > +
> > 
> > Please add a note about the locking requirements here, and for the other
> > new functions you've added.
> > 
> > You call this function from two places:
> > 
> > 1) fsldma_cleanup_descriptor() - called with mod->desc_lock held
> > 2) fsl_tx_status() - WITHOUT mod->desc_lock held
> > 
> > One of them is definitely wrong, and I'd bet that it is #2. You're
> > modifying ld_completed without a lock.
> Yes, My bad, I will correct it.
> 
> > 
> > > +static int
> > > +fsldma_clean_completed_descriptor(struct fsldma_chan *chan) {
> > > +	struct fsl_desc_sw *desc, *_desc;
> > > +
> > > +	/* Run the callback for each descriptor, in order */
> > > +	list_for_each_entry_safe(desc, _desc, &chan->ld_completed, node) {
> > > +
> > > +		if (async_tx_test_ack(&desc->async_tx)) {
> > > +			/* Remove from the list of transactions */
> > > +			list_del(&desc->node);
> > > +#ifdef FSL_DMA_LD_DEBUG
> > > +			chan_dbg(chan, "LD %p free\n", desc); #endif
> > > +			dma_pool_free(chan->desc_pool, desc,
> > > +					desc->async_tx.phys);
> > > +		}
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +/**
> > > + * fsldma_run_tx_complete_actions - cleanup and free a single link
> > > +descriptor
> > > + * @chan: Freescale DMA channel
> > > + * @desc: descriptor to cleanup and free
> > > + * @cookie: Freescale DMA transaction identifier
> > > + *
> > > + * This function is used on a descriptor which has been executed by
> > > +the DMA
> > > + * controller. It will run any callbacks, submit any dependencies,
> > > +and then
> > > + * free the descriptor.
> > > + */
> > > +static dma_cookie_t fsldma_run_tx_complete_actions(struct fsl_desc_sw
> > *desc,
> > > +		struct fsldma_chan *chan, dma_cookie_t cookie) {
> > > +	struct dma_async_tx_descriptor *txd = &desc->async_tx;
> > > +	struct device *dev = chan->common.device->dev;
> > > +	dma_addr_t src = get_desc_src(chan, desc);
> > > +	dma_addr_t dst = get_desc_dst(chan, desc);
> > > +	u32 len = get_desc_cnt(chan, desc);
> > > +
> > > +	BUG_ON(txd->cookie < 0);
> > > +
> > > +	if (txd->cookie > 0) {
> > > +		cookie = txd->cookie;
> > > +
> > > +		/* Run the link descriptor callback function */
> > > +		if (txd->callback) {
> > > +#ifdef FSL_DMA_LD_DEBUG
> > > +			chan_dbg(chan, "LD %p callback\n", desc); #endif
> > > +			txd->callback(txd->callback_param);
> > > +		}
> > > +
> > > +		/* Unmap the dst buffer, if requested */
> > > +		if (!(txd->flags & DMA_COMPL_SKIP_DEST_UNMAP)) {
> > > +			if (txd->flags & DMA_COMPL_DEST_UNMAP_SINGLE)
> > > +				dma_unmap_single(dev, dst, len, DMA_FROM_DEVICE);
> > > +			else
> > > +				dma_unmap_page(dev, dst, len, DMA_FROM_DEVICE);
> > > +		}
> > > +
> > > +		/* Unmap the src buffer, if requested */
> > > +		if (!(txd->flags & DMA_COMPL_SKIP_SRC_UNMAP)) {
> > > +			if (txd->flags & DMA_COMPL_SRC_UNMAP_SINGLE)
> > > +				dma_unmap_single(dev, src, len, DMA_TO_DEVICE);
> > > +			else
> > > +				dma_unmap_page(dev, src, len, DMA_TO_DEVICE);
> > > +		}
> > > +	}
> > > +
> > > +	/* Run any dependencies */
> > > +	dma_run_dependencies(txd);
> > > +
> > > +	return cookie;
> > > +}
> > > +
> > > +static int
> > > +fsldma_clean_running_descriptor(struct fsl_desc_sw *desc,
> > > +			struct fsldma_chan *chan)
> > > +{
> > > +	/* Remove from the list of transactions */
> > > +	list_del(&desc->node);
> > > +	/* the client is allowed to attach dependent operations
> > > +	 * until 'ack' is set
> > > +	 */
> > 
> > This comment is does not follow the coding style. It should be:
> > 
> > /*
> >  * the client is allowed to attech dependent operations
> >  * until 'ack' is set
> >  */
> Fine, I will correct it in v4. Include another 2 places related to coding style.
> Thanks.
> 
> > 
> > > +	if (!async_tx_test_ack(&desc->async_tx)) {
> > > +		/* move this slot to the completed */
> > 
> > Perhaps a better comment would be:
> > 
> > Move this descriptor to the list of descriptors which is complete, but
> > still awaiting the 'ack' bit to be set.
> Accept.
> 
> > 
> > > +		list_add_tail(&desc->node, &chan->ld_completed);
> > > +		return 0;
> > > +	}
> > > +
> > > +	dma_pool_free(chan->desc_pool, desc,
> > > +			desc->async_tx.phys);
> > 
> > This should all be on one line. It is less than 80 characters wide.
> > 
> Accept.
> 
> > > +	return 0;
> > > +}
> > > +
> > 
> > Locking notes please.
> I will add it in v4, please check. Thanks.
> 
> > 
> > > +static void fsldma_cleanup_descriptor(struct fsldma_chan *chan) {
> > > +	struct fsl_desc_sw *desc, *_desc;
> > > +	dma_cookie_t cookie = 0;
> > > +	dma_addr_t curr_phys = get_cdar(chan);
> > > +	int idle = dma_is_idle(chan);
> > > +	int seen_current = 0;
> > > +
> > > +	fsldma_clean_completed_descriptor(chan);
> > > +
> > > +	/* Run the callback for each descriptor, in order */
> > > +	list_for_each_entry_safe(desc, _desc, &chan->ld_running, node) {
> > > +		/* do not advance past the current descriptor loaded into the
> > > +		 * hardware channel, subsequent descriptors are either in
> > > +		 * process or have not been submitted
> > > +		 */
> > 
> > Coding style.
> > 
> > > +		if (seen_current)
> > > +			break;
> > > +
> > > +		/* stop the search if we reach the current descriptor and the
> > > +		 * channel is busy
> > > +		 */
> > 
> > Coding style.
> > 
> > > +		if (desc->async_tx.phys == curr_phys) {
> > > +			seen_current = 1;
> > > +			if (!idle)
> > > +				break;
> > > +		}
> > > +
> > 
> > This trick where you try to look at the hardware state to determine how
> > much to clean up has been a source of headaches in the past versions of
> > this driver.
> I know a little about the history of this driver. Could you tell me what kind
> headaches in the past versions, I can have a test and fix it in my patch. And
> I also think use current phys addr should be more explicit.
> Thanks.
> 

It has been a very long time since I last worked on this code, but I
will try to remember.

I remember one problem where the currently running descriptor was free'd
while the hardware was still executing it.

There was another related problem where the DMA hardware would lock up,
and it is impossible to recover without a hard reset. The CB (channel
busy) bit gets stuck on, and the running transfer never completes. This
may have been due to a programming issue in a user of the DMAEngine API,
and not this driver itself. I don't remember for sure anymore.

Our current system here at work has 120 boards with this DMA controller.
Each one runs 100+ DMA operations per second, 24/7/365. They've been
online for more than two years. I know for sure that the current code
doesn't lock up the DMA controller. :)

I think your bug is real, and needs to be fixed. I'm just trying to make
sure I understand the change, so it won't break other users. I'm only
trying to help.

> > 
> > It is much easier to reason about the state of the hardware and avoid
> > race conditions if you complete entire blocks of descriptors after the
> > hardware interrupts you to tell you it is finished.
> In current driver, it's ok, but there is a potential issue when enable
> async_tx api. How can you determine these descriptors in ld_running whether
> have been completed in fsl_tx_status()? (I mean in my patch.)
> 

I think your idea using an ld_completed list for descriptors which have
been run by the hardware, but not yet 'ack'ed by software is fine.

Would something similar to the following work?

ld_pending: build a list of descriptors until issue_pending() is called.
This is exactly what it does now, no changes needed.

ld_running: this list of descriptors is currently executing. Mostly
unchanged from how it works now.

When you get an interrupt, you know the descriptors in ld_running are
finished. Update the cookie to the highest number from ld_running. Free
the descriptors which have been 'ack'ed immediately. Move the ones which
are not 'ack'ed onto ld_completed. As part of this process, you should
run dma_run_dependencies() and callbacks as needed.

I think that this solves the problem you are seeing, which is that
descriptors which have not been 'ack'ed are free'd.

If this is unclear, I can write some code to demonstrate. If you could
write a small test driver, similar to drivers/dma/dmatest.c, which tests
the async_tx API without any extra hardware needed, I can run it. I have
never used the async_tx API before.

> > 
> > This is the philosophy employed by the driver before your modifications:
> > ld_pending: a block of descriptors which is ready to run as soon as the
> > hardware becomes idle.
> > ld_running: a block of descriptors which is being executed by the
> > hardware.
> These 2 lists are not enough, async_tx descriptors must be kept order as
> expected of its submitter, we only can process the descriptor which has been
> acked by async_tx api, but not free all descriptors in ld_running.
> For example,
> Step 1, in async_tx memcpy api,
> tx2 = device->device_prep_dma_memcpy(...),
> tx2->depend_tx = tx1;
> step 2, in async_tx submit api,
> async_tx_submit() will check tx2->depend_tx whether has been acked,
> unfortunately, tx2->depend_tx is freed before step 2 (dma channels is flushed
> by other drivers), the contents of tx2->depend_tx is set to POOL_POISON_FREED
> if DMAPOOL_DEBUG is enabled (illegal pointer will be used if disable this config
> option);
> step 3, BUG_ON(async_tx_test_ack(depend_tx)) is triggered.
> 
> For resolve this potential risk, first, ack flag must be checked in dma driver,
> second, ld_completed is added to restore all descriptors which have been acked
> and completed.
> In my following answer, most of all are around this idea.
> 
> > 
> > > +		cookie = fsldma_run_tx_complete_actions(desc, chan, cookie);
> > > +
> > > +		if (fsldma_clean_running_descriptor(desc, chan))
> > > +			break;
> > > +
> > > +	}
> > > +
> > > +	/*
> > > +	 * Start any pending transactions automatically
> > > +	 *
> > > +	 * In the ideal case, we keep the DMA controller busy while we go
> > > +	 * ahead and free the descriptors below.
> > > +	 */
> > > +	fsl_chan_xfer_ld_queue(chan);
> > > +
> > > +	if (cookie > 0)
> > > +		chan->common.completed_cookie = cookie; }
> > > +
> > >  static dma_cookie_t fsl_dma_tx_submit(struct dma_async_tx_descriptor
> > > *tx)  {
> > >  	struct fsldma_chan *chan = to_fsl_chan(tx->chan); @@ -534,8 +745,10
> > > @@ static void fsl_dma_free_chan_resources(struct dma_chan *dchan)
> > >
> > >  	chan_dbg(chan, "free all channel resources\n");
> > >  	spin_lock_irqsave(&chan->desc_lock, flags);
> > > +	fsldma_cleanup_descriptor(chan);
> > >  	fsldma_free_desc_list(chan, &chan->ld_pending);
> > >  	fsldma_free_desc_list(chan, &chan->ld_running);
> > > +	fsldma_free_desc_list(chan, &chan->ld_completed);
> > >  	spin_unlock_irqrestore(&chan->desc_lock, flags);
> > >
> > >  	dma_pool_destroy(chan->desc_pool);
> > > @@ -811,124 +1024,6 @@ static int fsl_dma_device_control(struct
> > > dma_chan *dchan,  }
> > >
> > >  /**
> > > - * fsldma_cleanup_descriptor - cleanup and free a single link
> > > descriptor
> > > - * @chan: Freescale DMA channel
> > > - * @desc: descriptor to cleanup and free
> > > - *
> > > - * This function is used on a descriptor which has been executed by
> > > the DMA
> > > - * controller. It will run any callbacks, submit any dependencies,
> > > and then
> > > - * free the descriptor.
> > > - */
> > > -static void fsldma_cleanup_descriptor(struct fsldma_chan *chan,
> > > -				      struct fsl_desc_sw *desc)
> > > -{
> > > -	struct dma_async_tx_descriptor *txd = &desc->async_tx;
> > > -	struct device *dev = chan->common.device->dev;
> > > -	dma_addr_t src = get_desc_src(chan, desc);
> > > -	dma_addr_t dst = get_desc_dst(chan, desc);
> > > -	u32 len = get_desc_cnt(chan, desc);
> > > -
> > > -	/* Run the link descriptor callback function */
> > > -	if (txd->callback) {
> > > -#ifdef FSL_DMA_LD_DEBUG
> > > -		chan_dbg(chan, "LD %p callback\n", desc);
> > > -#endif
> > > -		txd->callback(txd->callback_param);
> > > -	}
> > > -
> > > -	/* Run any dependencies */
> > > -	dma_run_dependencies(txd);
> > > -
> > > -	/* Unmap the dst buffer, if requested */
> > > -	if (!(txd->flags & DMA_COMPL_SKIP_DEST_UNMAP)) {
> > > -		if (txd->flags & DMA_COMPL_DEST_UNMAP_SINGLE)
> > > -			dma_unmap_single(dev, dst, len, DMA_FROM_DEVICE);
> > > -		else
> > > -			dma_unmap_page(dev, dst, len, DMA_FROM_DEVICE);
> > > -	}
> > > -
> > > -	/* Unmap the src buffer, if requested */
> > > -	if (!(txd->flags & DMA_COMPL_SKIP_SRC_UNMAP)) {
> > > -		if (txd->flags & DMA_COMPL_SRC_UNMAP_SINGLE)
> > > -			dma_unmap_single(dev, src, len, DMA_TO_DEVICE);
> > > -		else
> > > -			dma_unmap_page(dev, src, len, DMA_TO_DEVICE);
> > > -	}
> > > -
> > > -#ifdef FSL_DMA_LD_DEBUG
> > > -	chan_dbg(chan, "LD %p free\n", desc);
> > > -#endif
> > > -	dma_pool_free(chan->desc_pool, desc, txd->phys);
> > > -}
> > > -
> > > -/**
> > > - * fsl_chan_xfer_ld_queue - transfer any pending transactions
> > > - * @chan : Freescale DMA channel
> > > - *
> > > - * HARDWARE STATE: idle
> > > - * LOCKING: must hold chan->desc_lock
> > > - */
> > > -static void fsl_chan_xfer_ld_queue(struct fsldma_chan *chan) -{
> > > -	struct fsl_desc_sw *desc;
> > > -
> > > -	/*
> > > -	 * If the list of pending descriptors is empty, then we
> > > -	 * don't need to do any work at all
> > > -	 */
> > > -	if (list_empty(&chan->ld_pending)) {
> > > -		chan_dbg(chan, "no pending LDs\n");
> > > -		return;
> > > -	}
> > > -
> > > -	/*
> > > -	 * The DMA controller is not idle, which means that the interrupt
> > > -	 * handler will start any queued transactions when it runs after
> > > -	 * this transaction finishes
> > > -	 */
> > > -	if (!chan->idle) {
> > > -		chan_dbg(chan, "DMA controller still busy\n");
> > > -		return;
> > > -	}
> > > -
> > > -	/*
> > > -	 * If there are some link descriptors which have not been
> > > -	 * transferred, we need to start the controller
> > > -	 */
> > > -
> > > -	/*
> > > -	 * Move all elements from the queue of pending transactions
> > > -	 * onto the list of running transactions
> > > -	 */
> > > -	chan_dbg(chan, "idle, starting controller\n");
> > > -	desc = list_first_entry(&chan->ld_pending, struct fsl_desc_sw,
> > node);
> > > -	list_splice_tail_init(&chan->ld_pending, &chan->ld_running);
> > > -
> > > -	/*
> > > -	 * The 85xx DMA controller doesn't clear the channel start bit
> > > -	 * automatically at the end of a transfer. Therefore we must clear
> > > -	 * it in software before starting the transfer.
> > > -	 */
> > > -	if ((chan->feature & FSL_DMA_IP_MASK) == FSL_DMA_IP_85XX) {
> > > -		u32 mode;
> > > -
> > > -		mode = DMA_IN(chan, &chan->regs->mr, 32);
> > > -		mode &= ~FSL_DMA_MR_CS;
> > > -		DMA_OUT(chan, &chan->regs->mr, mode, 32);
> > > -	}
> > > -
> > > -	/*
> > > -	 * Program the descriptor's address into the DMA controller,
> > > -	 * then start the DMA transaction
> > > -	 */
> > > -	set_cdar(chan, desc->async_tx.phys);
> > > -	get_cdar(chan);
> > > -
> > > -	dma_start(chan);
> > > -	chan->idle = false;
> > > -}
> > > -
> > > -/**
> > >   * fsl_dma_memcpy_issue_pending - Issue the DMA start command
> > >   * @chan : Freescale DMA channel
> > >   */
> > > @@ -954,11 +1049,17 @@ static enum dma_status fsl_tx_status(struct
> > dma_chan *dchan,
> > >  	enum dma_status ret;
> > >  	unsigned long flags;
> > >
> > > -	spin_lock_irqsave(&chan->desc_lock, flags);
> > >  	ret = dma_cookie_status(dchan, cookie, txstate);
> > > +	if (ret == DMA_SUCCESS) {
> > > +		fsldma_clean_completed_descriptor(chan);
> > > +		return ret;
> > > +	}
> > > +
> > > +	spin_lock_irqsave(&chan->desc_lock, flags);
> > > +	fsldma_cleanup_descriptor(chan);
> > 
> > You've made status from a very quick operation (compare two cookies) into
> > a potentially long running operation. It may now run callbacks, unmap
> > pages, etc.
> Yes, that's right, callbacks and unmap pages will be invoked if DMA status
> is not DMA_SUCCESS.
> 
> > 
> > I note that Documentation/crypto/async-tx-api.txt section 3.6
> > "Constraints" does say that async_*() functions cannot be called from IRQ
> > context. However, the DMAEngine API itself does not have anything to say
> > about IRQ context.
> > 
> > I expect fsl_tx_status() to be quick, and not potentially call other
> > arbitrary code.
> fsl_tx_status() is not in IRQ context. It's reasonable to unmap pages and
> run dependency of descriptor as fsldma_run_tx_complete_actions() does.
> 

I looked at several other DMAEngine API drivers. Some of them do run
dependencies and callbacks from tx_status(). You are correct, this is
fine.

> > 
> > Is it possible to leave this function unchanged?
> According to my knowledge, it is unlikely to be left unchanged, or it will violate
> the design of this interface. If DMA status is not DMA_SUCCESS, that means there
> are new descriptors completed, we should move them to ld_completed, and do actions
> to its async_tx descriptors, then dma descriptor will be freed at another time.
> Most of my idea is from iop-adma.c, the original interface is not align with it.
> Async_tx flags (expecially ack_test) is not considered when dma descriptor is freed,
> so some random errors happened, I think you must familiar with this history.
> As an important "synchronization flag", async_tx api must control the order of tx
> descriptor. Dma driver also should make sure keeping order when free the descriptor.
> That's the reason why I add ld_completed list.
> 

Is it possible to put the descriptors into three different states?

1) ld_pending: ready to run, but not yet executed on the hardware

Descriptors move to #2 through either dma_async_issue_pending() or by
the interrupt which happens when the hardware completes executing a set
of DMA descriptors.

2) ld_running: currently executing on the hardware

Descriptors move to #3 through the interrupt which happens when the
hardware completes executing a set of DMA descriptors.

Dependencies and callbacks are executed as the descriptors are moved
onto ld_completed.

3) ld_completed: finished executing on the hardware, but not yet 'ack'ed

When they have been 'ack'ed, the descriptors are free'd.

As noted above, I think I could code this up fairly quickly if this
explanation is unclear.

Also note that I may not understand your bug completely, and my idea may
be completely wrong!

Ira

> > 
> > Also note that if you leave this function unchanged, the locking error
> > pointed out above (fsldma_clean_completed_descriptor() is not called with
> > the lock held) goes away.
> I know. Thanks.
> 
> > 
> > >  	spin_unlock_irqrestore(&chan->desc_lock, flags);
> > >
> > > -	return ret;
> > > +	return dma_cookie_status(dchan, cookie, txstate);
> > >  }
> > >
> > >
> > > /*--------------------------------------------------------------------
> > > --------*/ @@ -1035,53 +1136,21 @@ static irqreturn_t
> > > fsldma_chan_irq(int irq, void *data)  static void
> > > dma_do_tasklet(unsigned long data)  {
> > >  	struct fsldma_chan *chan = (struct fsldma_chan *)data;
> > > -	struct fsl_desc_sw *desc, *_desc;
> > > -	LIST_HEAD(ld_cleanup);
> > >  	unsigned long flags;
> > >
> > >  	chan_dbg(chan, "tasklet entry\n");
> > >
> > >  	spin_lock_irqsave(&chan->desc_lock, flags);
> > >
> > > -	/* update the cookie if we have some descriptors to cleanup */
> > > -	if (!list_empty(&chan->ld_running)) {
> > > -		dma_cookie_t cookie;
> > > -
> > > -		desc = to_fsl_desc(chan->ld_running.prev);
> > > -		cookie = desc->async_tx.cookie;
> > > -		dma_cookie_complete(&desc->async_tx);
> > > -
> > > -		chan_dbg(chan, "completed_cookie=%d\n", cookie);
> > > -	}
> > > -
> > > -	/*
> > > -	 * move the descriptors to a temporary list so we can drop the lock
> > > -	 * during the entire cleanup operation
> > > -	 */
> > > -	list_splice_tail_init(&chan->ld_running, &ld_cleanup);
> > > +	/* Run all cleanup for this descriptor */
> > > +	fsldma_cleanup_descriptor(chan);
> > >
> > >  	/* the hardware is now idle and ready for more */
> > >  	chan->idle = true;
> > >
> > > -	/*
> > > -	 * Start any pending transactions automatically
> > > -	 *
> > > -	 * In the ideal case, we keep the DMA controller busy while we go
> > > -	 * ahead and free the descriptors below.
> > > -	 */
> > >  	fsl_chan_xfer_ld_queue(chan);
> > >  	spin_unlock_irqrestore(&chan->desc_lock, flags);
> > >
> > > -	/* Run the callback for each descriptor, in order */
> > > -	list_for_each_entry_safe(desc, _desc, &ld_cleanup, node) {
> > > -
> > > -		/* Remove from the list of transactions */
> > > -		list_del(&desc->node);
> > > -
> > > -		/* Run all cleanup for this descriptor */
> > > -		fsldma_cleanup_descriptor(chan, desc);
> > > -	}
> > > -
> > >  	chan_dbg(chan, "tasklet exit\n");
> > >  }
> > >
> > > @@ -1262,6 +1331,7 @@ static int __devinit fsl_dma_chan_probe(struct
> > fsldma_device *fdev,
> > >  	spin_lock_init(&chan->desc_lock);
> > >  	INIT_LIST_HEAD(&chan->ld_pending);
> > >  	INIT_LIST_HEAD(&chan->ld_running);
> > > +	INIT_LIST_HEAD(&chan->ld_completed);
> > >  	chan->idle = true;
> > >
> > >  	chan->common.device = &fdev->common; diff --git
> > > a/drivers/dma/fsldma.h b/drivers/dma/fsldma.h index f5c3879..7ede908
> > > 100644
> > > --- a/drivers/dma/fsldma.h
> > > +++ b/drivers/dma/fsldma.h
> > > @@ -140,6 +140,7 @@ struct fsldma_chan {
> > >  	spinlock_t desc_lock;		/* Descriptor operation lock */
> > >  	struct list_head ld_pending;	/* Link descriptors queue */
> > >  	struct list_head ld_running;	/* Link descriptors queue */
> > > +	struct list_head ld_completed;	/* Link descriptors queue */
> > >  	struct dma_chan common;		/* DMA common channel */
> > >  	struct dma_pool *desc_pool;	/* Descriptors pool */
> > >  	struct device *dev;		/* Channel device */
> > > --
> > > 1.7.5.1
> > >
> > >
> 
>