Re: [PATCH v4 2/9] dmaengine: Add safe API to combine configuration and preparation

On Wed, May 06, 2026 at 04:44:14PM -0400, Frank Li wrote:
> Introduce dmaengine_prep_config_single_safe() and
> dmaengine_prep_config_sg_safe() to provide a reentrant-safe way to
> combine slave configuration and transfer preparation.
> 
> Drivers may implement the new device_prep_config_sg() callback to perform
> both steps atomically. If the callback is not provided, the helpers fall
> back to calling dmaengine_slave_config() followed by
> dmaengine_prep_slave_sg() under per-channel mutex protection.
> 
> Tested-by: Niklas Cassel <[email protected]>
> Signed-off-by: Frank Li <[email protected]>
> ---
> chagne in v4
> - use spinlock() to protect config() and prep()
> 
> change in v3
> - new patch
> ---
>  drivers/dma/dmaengine.c   |  2 ++
>  include/linux/dmaengine.h | 58 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 60 insertions(+)
> 
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 405bd2fbb4a3b94fd0bf44526f656f6a19feaad0..ba29e60160c1a0148793bb299849bccfebb6d32b 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -1099,6 +1099,8 @@ static int __dma_async_device_channel_register(struct dma_device *device,
>  	chan->dev->device.parent = device->dev;
>  	chan->dev->chan = chan;
>  	chan->dev->dev_id = device->dev_id;
> +	spin_lock_init(&chan->lock);
> +
>  	if (!name)
>  		dev_set_name(&chan->dev->device, "dma%dchan%d", device->dev_id, chan->chan_id);
>  	else
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index defa377d2ef54d94e6337cdfa7826a091295535e..23728f3d60804e49cd4cbbd3a513c4936eed5836 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -322,6 +322,8 @@ struct dma_router {
>   * @slave: ptr to the device using this channel
>   * @cookie: last cookie value returned to client
>   * @completed_cookie: last completed cookie for this channel
> + * @lock: protect between config and prepare transfer when driver have not
> + *	  implemented callback device_prep_config_sg().
>   * @chan_id: channel ID for sysfs
>   * @dev: class device for sysfs
>   * @name: backlink name for sysfs
> @@ -341,6 +343,12 @@ struct dma_chan {
>  	dma_cookie_t cookie;
>  	dma_cookie_t completed_cookie;
>  
> +	/*
> +	 * protect between config and prepare transfer because *_prep() may be
> +	 * called from complete callback, which is in GFP_NOSLEEP context.
> +	 */
> +	spinlock_t lock; /* protect between config and prepare transfer since */

Why two comments?

> +
>  	/* sysfs */
>  	int chan_id;
>  	struct dma_chan_dev *dev;
> @@ -1068,6 +1076,56 @@ dmaengine_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl,
>  	return dmaengine_prep_config_sg(chan, sgl, sg_len, dir, flags, NULL);
>  }
>  
> +/*
> + * dmaengine_prep_config_single(sg)_safe() is re-entrant version.
> + *
> + * The unsafe variant (without the _safe suffix) falls back to calling
> + * dmaengine_slave_config() and dmaengine_prep_slave_sg() separately.
> + * In this case, additional locking may be required, depending on the
> + * DMA consumer's usage.
> + *
> + * If dmaengine driver have not implemented call back device_prep_config_sg()
> + * safe version use per-channel spinlock to protect call dmaengine_slave_config()
> + * and dmaengine_prep_slave_sg().
> + */

Use proper kernel-doc comments please...

> +static inline struct dma_async_tx_descriptor *
> +dmaengine_prep_config_sg_safe(struct dma_chan *chan, struct scatterlist *sgl,
> +			      unsigned int sg_len,
> +			      enum dma_transfer_direction dir,
> +			      unsigned long flags,
> +			      struct dma_slave_config *config)
> +{
> +	struct dma_async_tx_descriptor *tx;
> +
> +	if (!chan || !chan->device)
> +		return NULL;
> +
> +	if (!chan->device->device_prep_config_sg)
> +		spin_lock(&chan->lock);
> +
> +	tx = dmaengine_prep_config_sg(chan, sgl, sg_len, dir, flags, config);
> +
> +	if (!chan->device->device_prep_config_sg)
> +		spin_unlock(&chan->lock);
> +
> +	return tx;
> +}
> +

Missing kernel-doc.

- Mani

-- 
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