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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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