Re: [PATCH v2 4/4] dmaengine: dma-axi-dmac: Defer freeing DMA descriptors

[Frank Li]

On Fri, Mar 27, 2026 at 04:58:41PM +0000, Nuno Sá wrote:
> From: Eliza Balas <eliza.balas@xxxxxxxxxx>
>
> This IP core can be used in architectures (like Microblaze) where DMA
> descriptors are allocated with vmalloc().

strage, why use vmalloc()?

Frank

>  Hence, given that freeing the
> descriptors happen in softirq context, vunmpap() will BUG().
>
> To solve the above, we setup a work item during allocation of the
> descriptors and schedule in softirq context. Hence, the actual freeing
> happens in threaded context.
>
> Also note that to account for the possible race where the struct axi_dmac
> object is gone between scheduling the work and actually running it, we
> now save and get a reference of struct device when allocating the
> descriptor (given that's all we need in axi_dmac_free_desc()) and
> release it in axi_dmac_free_desc().
>
> Signed-off-by: Eliza Balas <eliza.balas@xxxxxxxxxx>
> Co-developed-by: Nuno Sá <nuno.sa@xxxxxxxxxx>
> Signed-off-by: Nuno Sá <nuno.sa@xxxxxxxxxx>
> ---
>  drivers/dma/dma-axi-dmac.c | 50 ++++++++++++++++++++++++++++++++++------------
>  1 file changed, 37 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/dma/dma-axi-dmac.c b/drivers/dma/dma-axi-dmac.c
> index 70d3ad7e7d37..46f1ead0c7d7 100644
> --- a/drivers/dma/dma-axi-dmac.c
> +++ b/drivers/dma/dma-axi-dmac.c
> @@ -25,6 +25,7 @@
>  #include <linux/regmap.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
> +#include <linux/workqueue.h>
>
>  #include <dt-bindings/dma/axi-dmac.h>
>
> @@ -133,6 +134,9 @@ struct axi_dmac_sg {
>  struct axi_dmac_desc {
>  	struct virt_dma_desc vdesc;
>  	struct axi_dmac_chan *chan;
> +	struct device *dev;
> +
> +	struct work_struct sched_work;
>
>  	bool cyclic;
>  	bool cyclic_eot;
> @@ -666,6 +670,25 @@ static void axi_dmac_issue_pending(struct dma_chan *c)
>  	spin_unlock_irqrestore(&chan->vchan.lock, flags);
>  }
>
> +static void axi_dmac_free_desc(struct axi_dmac_desc *desc)
> +{
> +	struct axi_dmac_hw_desc *hw = desc->sg[0].hw;
> +	dma_addr_t hw_phys = desc->sg[0].hw_phys;
> +
> +	dma_free_coherent(desc->dev, PAGE_ALIGN(desc->num_sgs * sizeof(*hw)),
> +			  hw, hw_phys);
> +	put_device(desc->dev);
> +	kfree(desc);
> +}
> +
> +static void axi_dmac_free_desc_schedule_work(struct work_struct *work)
> +{
> +	struct axi_dmac_desc *desc = container_of(work, struct axi_dmac_desc,
> +						  sched_work);
> +
> +	axi_dmac_free_desc(desc);
> +}
> +
>  static struct axi_dmac_desc *
>  axi_dmac_alloc_desc(struct axi_dmac_chan *chan, unsigned int num_sgs)
>  {
> @@ -681,6 +704,7 @@ axi_dmac_alloc_desc(struct axi_dmac_chan *chan, unsigned int num_sgs)
>  		return NULL;
>  	desc->num_sgs = num_sgs;
>  	desc->chan = chan;
> +	desc->dev = get_device(dmac->dma_dev.dev);
>
>  	hws = dma_alloc_coherent(dev, PAGE_ALIGN(num_sgs * sizeof(*hws)),
>  				&hw_phys, GFP_ATOMIC);
> @@ -703,21 +727,18 @@ axi_dmac_alloc_desc(struct axi_dmac_chan *chan, unsigned int num_sgs)
>  	/* The last hardware descriptor will trigger an interrupt */
>  	desc->sg[num_sgs - 1].hw->flags = AXI_DMAC_HW_FLAG_LAST | AXI_DMAC_HW_FLAG_IRQ;
>
> +	/*
> +	 * We need to setup a work item because this IP can be used on archs
> +	 * that rely on vmalloced memory for descriptors. And given that freeing
> +	 * the descriptors happens in softirq context, vunmpap() will BUG().
> +	 * Hence, setup the worker so that we can queue it and free the
> +	 * descriptor in threaded context.
> +	 */
> +	INIT_WORK(&desc->sched_work, axi_dmac_free_desc_schedule_work);
> +
>  	return desc;
>  }
>
> -static void axi_dmac_free_desc(struct axi_dmac_desc *desc)
> -{
> -	struct axi_dmac *dmac = chan_to_axi_dmac(desc->chan);
> -	struct device *dev = dmac->dma_dev.dev;
> -	struct axi_dmac_hw_desc *hw = desc->sg[0].hw;
> -	dma_addr_t hw_phys = desc->sg[0].hw_phys;
> -
> -	dma_free_coherent(dev, PAGE_ALIGN(desc->num_sgs * sizeof(*hw)),
> -			  hw, hw_phys);
> -	kfree(desc);
> -}
> -
>  static struct axi_dmac_sg *axi_dmac_fill_linear_sg(struct axi_dmac_chan *chan,
>  	enum dma_transfer_direction direction, dma_addr_t addr,
>  	unsigned int num_periods, unsigned int period_len,
> @@ -958,7 +979,10 @@ static void axi_dmac_free_chan_resources(struct dma_chan *c)
>
>  static void axi_dmac_desc_free(struct virt_dma_desc *vdesc)
>  {
> -	axi_dmac_free_desc(to_axi_dmac_desc(vdesc));
> +	struct axi_dmac_desc *desc = to_axi_dmac_desc(vdesc);
> +
> +	/* See the comment in axi_dmac_alloc_desc() for the why! */
> +	schedule_work(&desc->sched_work);
>  }
>
>  static bool axi_dmac_regmap_rdwr(struct device *dev, unsigned int reg)
>
> --
> 2.53.0
>