linux-kernel - [PATCH v3 2/9] dmaengine: Add safe API to combine configuration and preparation

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Message-Id: <20260105-dma_prep_config-v3-2-a8480362fd42@nxp.com>
Date: Mon, 05 Jan 2026 17:46:52 -0500
From: Frank Li <Frank.Li@....com>
To: Vinod Koul <vkoul@...nel.org>, Manivannan Sadhasivam <mani@...nel.org>, 
 Krzysztof Wilczyński <kwilczynski@...nel.org>, 
 Kishon Vijay Abraham I <kishon@...nel.org>, 
 Bjorn Helgaas <bhelgaas@...gle.com>, Christoph Hellwig <hch@....de>, 
 Sagi Grimberg <sagi@...mberg.me>, Chaitanya Kulkarni <kch@...dia.com>, 
 Herbert Xu <herbert@...dor.apana.org.au>, 
 "David S. Miller" <davem@...emloft.net>, 
 Nicolas Ferre <nicolas.ferre@...rochip.com>, 
 Alexandre Belloni <alexandre.belloni@...tlin.com>, 
 Claudiu Beznea <claudiu.beznea@...on.dev>, Koichiro Den <den@...inux.co.jp>, 
 Niklas Cassel <cassel@...nel.org>
Cc: dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org, 
 mhi@...ts.linux.dev, linux-arm-msm@...r.kernel.org, 
 linux-crypto@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
 imx@...ts.linux.dev, Frank Li <Frank.Li@....com>
Subject: [PATCH v3 2/9] dmaengine: Add safe API to combine configuration
 and preparation

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.

Signed-off-by: Frank Li <Frank.Li@....com>
---
change in v3
- new patch
---
 drivers/dma/dmaengine.c   |  3 +++
 include/linux/dmaengine.h | 53 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index ca13cd39330ba4d822baaab412356a166b656350..53116300e61078ca89e78109bcf24a5f8c7e3369 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -1097,6 +1097,8 @@ 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;
+	mutex_init(&chan->lock);
+
 	if (!name)
 		dev_set_name(&chan->dev->device, "dma%dchan%d", device->dev_id, chan->chan_id);
 	else
@@ -1147,6 +1149,7 @@ static void __dma_async_device_channel_unregister(struct dma_device *device,
 	device->chancnt--;
 	chan->dev->chan = NULL;
 	mutex_unlock(&dma_list_mutex);
+	mutex_destroy(&chan->lock);
 	ida_free(&device->chan_ida, chan->chan_id);
 	device_unregister(&chan->dev->device);
 	free_percpu(chan->local);
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 4994236aaadc45dbda260b63abe1fef47aa3d51e..abb4a7424a0083c00730a945c1cb645f831fbd6f 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
@@ -340,6 +342,7 @@ struct dma_chan {
 	struct device *slave;
 	dma_cookie_t cookie;
 	dma_cookie_t completed_cookie;
+	struct mutex lock; /* protect between config and prepare transfer */
 
 	/* sysfs */
 	int chan_id;
@@ -1068,6 +1071,56 @@ 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 mutex to protect call dmaengine_slave_config()
+ * and dmaengine_prep_slave_sg().
+ */
+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)
+		mutex_lock(&chan->lock);
+
+	tx = dmaengine_prep_config_sg(chan, sgl, sg_len, dir, flags, config);
+
+	if (!chan->device->device_prep_config_sg)
+		mutex_unlock(&chan->lock);
+
+	return tx;
+}
+
+static inline struct dma_async_tx_descriptor *
+dmaengine_prep_config_single_safe(struct dma_chan *chan, dma_addr_t buf,
+				  size_t len, enum dma_transfer_direction dir,
+				  unsigned long flags,
+				  struct dma_slave_config *config)
+{
+	struct scatterlist sg;
+
+	sg_init_table(&sg, 1);
+	sg_dma_address(&sg) = buf;
+	sg_dma_len(&sg) = len;
+
+	return dmaengine_prep_config_sg_safe(chan, &sg, 1, dir, flags, config);
+}
+
 #ifdef CONFIG_RAPIDIO_DMA_ENGINE
 struct rio_dma_ext;
 static inline struct dma_async_tx_descriptor *dmaengine_prep_rio_sg(

-- 
2.34.1