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SubjectRe: [PATCH 02/13] dmaengine: remove dependency on async_tx
On Fri, 14 Nov 2008 14:34:27 -0700 Dan Williams <dan.j.williams@intel.com> wrote:

> async_tx.ko is a consumer of dma channels. A circular dependency arises
> if modules in drivers/dma rely on common code in async_tx.ko. It
> prevents either module from being unloaded.
>
> Move dma_wait_for_async_tx and async_tx_run_dependencies to dmaeninge.o
> where they should have been from the beginning.
>
>
> ...
>
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -623,6 +623,81 @@ void dma_async_tx_descriptor_init(struct dma_async_tx_descriptor *tx,
> }
> EXPORT_SYMBOL(dma_async_tx_descriptor_init);
>
> +/* dma_wait_for_async_tx - spin wait for a transcation to complete

yuo cnat sepll

> + * @tx: transaction to wait on
> + */
> +enum dma_status
> +dma_wait_for_async_tx(struct dma_async_tx_descriptor *tx)
> +{
> + enum dma_status status;
> + struct dma_async_tx_descriptor *iter;
> + struct dma_async_tx_descriptor *parent;
> +
> + if (!tx)
> + return DMA_SUCCESS;
> +
> + /* poll through the dependency chain, return when tx is complete */
> + do {
> + iter = tx;
> +
> + /* find the root of the unsubmitted dependency chain */
> + do {
> + parent = iter->parent;
> + if (!parent)
> + break;
> + else
> + iter = parent;
> + } while (parent);
> +
> + /* there is a small window for ->parent == NULL and
> + * ->cookie == -EBUSY
> + */
> + while (iter->cookie == -EBUSY)
> + cpu_relax();
> +
> + status = dma_sync_wait(iter->chan, iter->cookie);
> + } while (status == DMA_IN_PROGRESS || (iter != tx));
> +
> + return status;
> +}
> +EXPORT_SYMBOL_GPL(dma_wait_for_async_tx);

hm, strange.

The unlocked list walk assumes that this thread of control has
exclusive access to *tx (somehow??), but this thread of control doesn't
end up freeing *tx. I guess the caller does that.

> +/* dma_run_dependencies - helper routine for dma drivers to process
> + * (start) dependent operations on their target channel
> + * @tx: transaction with dependencies
> + */
> +void dma_run_dependencies(struct dma_async_tx_descriptor *tx)
> +{
> + struct dma_async_tx_descriptor *dep = tx->next;
> + struct dma_async_tx_descriptor *dep_next;
> + struct dma_chan *chan;
> +
> + if (!dep)
> + return;
> +
> + chan = dep->chan;
> +
> + /* keep submitting up until a channel switch is detected
> + * in that case we will be called again as a result of
> + * processing the interrupt from async_tx_channel_switch
> + */
> + for (; dep; dep = dep_next) {
> + spin_lock_bh(&dep->lock);
> + dep->parent = NULL;
> + dep_next = dep->next;
> + if (dep_next && dep_next->chan == chan)
> + dep->next = NULL; /* ->next will be submitted */
> + else
> + dep_next = NULL; /* submit current dep and terminate */
> + spin_unlock_bh(&dep->lock);
> +
> + dep->tx_submit(dep);
> + }
> +
> + chan->device->device_issue_pending(chan);
> +}
> +EXPORT_SYMBOL_GPL(dma_run_dependencies);

Now there's a lock.




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