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    Subject[PATCH RESEND 1/2] dmaengine: virt-dma: don't always free descriptor upon completion
    Date
    This patch attempts to enhance the case of a transfer submitted multiple
    times, and where the cost of creating the descriptors chain is not
    negligible.

    This happens with big video buffers (several megabytes, ie. several
    thousands of linked descriptors in one scatter-gather list). In these
    cases, a video driver would want to do :
    - tx = dmaengine_prep_slave_sg()
    - dma_engine_submit(tx);
    - dma_async_issue_pending()
    - wait for video completion
    - read video data (or not, skipping a frame is also possible)
    - dma_engine_submit(tx)
    => here, the descriptors chain recalculation will take time
    => the dma coherent allocation over and over might create holes in
    the dma pool, which is counter-productive.
    - dma_async_issue_pending()
    - etc ...

    In order to cope with this case, virt-dma is modified to prevent freeing
    the descriptors upon completion if DMA_CTRL_ACK flag is set in the
    transfer.

    Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
    Cc: Lars-Peter Clausen <lars@metafoo.de>
    Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
    ---
    drivers/dma/virt-dma.c | 19 +++++++++++++------
    drivers/dma/virt-dma.h | 13 ++++++++++++-
    2 files changed, 25 insertions(+), 7 deletions(-)

    diff --git a/drivers/dma/virt-dma.c b/drivers/dma/virt-dma.c
    index 6f80432..7d2c17d 100644
    --- a/drivers/dma/virt-dma.c
    +++ b/drivers/dma/virt-dma.c
    @@ -29,7 +29,7 @@ dma_cookie_t vchan_tx_submit(struct dma_async_tx_descriptor *tx)
    spin_lock_irqsave(&vc->lock, flags);
    cookie = dma_cookie_assign(tx);

    - list_add_tail(&vd->node, &vc->desc_submitted);
    + list_move_tail(&vd->node, &vc->desc_submitted);
    spin_unlock_irqrestore(&vc->lock, flags);

    dev_dbg(vc->chan.device->dev, "vchan %p: txd %p[%x]: submitted\n",
    @@ -83,8 +83,10 @@ static void vchan_complete(unsigned long arg)
    cb_data = vd->tx.callback_param;

    list_del(&vd->node);
    -
    - vc->desc_free(vd);
    + if (async_tx_test_ack(&vd->tx))
    + list_add(&vd->node, &vc->desc_allocated);
    + else
    + vc->desc_free(vd);

    if (cb)
    cb(cb_data);
    @@ -96,9 +98,13 @@ void vchan_dma_desc_free_list(struct virt_dma_chan *vc, struct list_head *head)
    while (!list_empty(head)) {
    struct virt_dma_desc *vd = list_first_entry(head,
    struct virt_dma_desc, node);
    - list_del(&vd->node);
    - dev_dbg(vc->chan.device->dev, "txd %p: freeing\n", vd);
    - vc->desc_free(vd);
    + if (async_tx_test_ack(&vd->tx)) {
    + list_move_tail(&vd->node, &vc->desc_allocated);
    + } else {
    + dev_dbg(vc->chan.device->dev, "txd %p: freeing\n", vd);
    + list_del(&vd->node);
    + vc->desc_free(vd);
    + }
    }
    }
    EXPORT_SYMBOL_GPL(vchan_dma_desc_free_list);
    @@ -108,6 +114,7 @@ void vchan_init(struct virt_dma_chan *vc, struct dma_device *dmadev)
    dma_cookie_init(&vc->chan);

    spin_lock_init(&vc->lock);
    + INIT_LIST_HEAD(&vc->desc_allocated);
    INIT_LIST_HEAD(&vc->desc_submitted);
    INIT_LIST_HEAD(&vc->desc_issued);
    INIT_LIST_HEAD(&vc->desc_completed);
    diff --git a/drivers/dma/virt-dma.h b/drivers/dma/virt-dma.h
    index 181b952..189e75d 100644
    --- a/drivers/dma/virt-dma.h
    +++ b/drivers/dma/virt-dma.h
    @@ -29,6 +29,7 @@ struct virt_dma_chan {
    spinlock_t lock;

    /* protected by vc.lock */
    + struct list_head desc_allocated;
    struct list_head desc_submitted;
    struct list_head desc_issued;
    struct list_head desc_completed;
    @@ -55,11 +56,16 @@ static inline struct dma_async_tx_descriptor *vchan_tx_prep(struct virt_dma_chan
    struct virt_dma_desc *vd, unsigned long tx_flags)
    {
    extern dma_cookie_t vchan_tx_submit(struct dma_async_tx_descriptor *);
    + unsigned long flags;

    dma_async_tx_descriptor_init(&vd->tx, &vc->chan);
    vd->tx.flags = tx_flags;
    vd->tx.tx_submit = vchan_tx_submit;

    + spin_lock_irqsave(&vc->lock, flags);
    + list_add_tail(&vd->node, &vc->desc_allocated);
    + spin_unlock_irqrestore(&vc->lock, flags);
    +
    return &vd->tx;
    }

    @@ -122,7 +128,8 @@ static inline struct virt_dma_desc *vchan_next_desc(struct virt_dma_chan *vc)
    }

    /**
    - * vchan_get_all_descriptors - obtain all submitted and issued descriptors
    + * vchan_get_all_descriptors - obtain all allocated, submitted and issued
    + * descriptors
    * vc: virtual channel to get descriptors from
    * head: list of descriptors found
    *
    @@ -134,6 +141,7 @@ static inline struct virt_dma_desc *vchan_next_desc(struct virt_dma_chan *vc)
    static inline void vchan_get_all_descriptors(struct virt_dma_chan *vc,
    struct list_head *head)
    {
    + list_splice_tail_init(&vc->desc_allocated, head);
    list_splice_tail_init(&vc->desc_submitted, head);
    list_splice_tail_init(&vc->desc_issued, head);
    list_splice_tail_init(&vc->desc_completed, head);
    @@ -141,11 +149,14 @@ static inline void vchan_get_all_descriptors(struct virt_dma_chan *vc,

    static inline void vchan_free_chan_resources(struct virt_dma_chan *vc)
    {
    + struct virt_dma_desc *vd;
    unsigned long flags;
    LIST_HEAD(head);

    spin_lock_irqsave(&vc->lock, flags);
    vchan_get_all_descriptors(vc, &head);
    + list_for_each_entry(vd, &head, node)
    + async_tx_clear_ack(&vd->tx);
    spin_unlock_irqrestore(&vc->lock, flags);

    vchan_dma_desc_free_list(vc, &head);
    --
    2.1.4


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