spi: add basic support for SPI offloading

Add the basic infrastructure to support SPI offload providers and
consumers.

SPI offloading is a feature that allows the SPI controller to perform
transfers without any CPU intervention. This is useful, e.g. for
high-speed data acquisition.

SPI controllers with offload support need to implement the get_offload
and put_offload callbacks and can use the devm_spi_offload_alloc() to
allocate offload instances.

SPI peripheral drivers will call devm_spi_offload_get() to get a
reference to the matching offload instance. This offload instance can
then be attached to a SPI message to request offloading that message.

It is expected that SPI controllers with offload support will check for
the offload instance in the SPI message in the ctlr->optimize_message()
callback and handle it accordingly.

CONFIG_SPI_OFFLOAD is intended to be a select-only option. Both
consumer and provider drivers should `select SPI_OFFLOAD` in their
Kconfig to ensure that the SPI core is built with offload support.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-1-e48a489be48c@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

MAINTAINERS

@@ -22295,6 +22295,12 @@ F:	Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
 F:	drivers/mtd/spi-nor/
 F:	include/linux/mtd/spi-nor.h
 
+SPI OFFLOAD
+R:	David Lechner <dlechner@baylibre.com>
+F:	drivers/spi/spi-offload.c
+F:	include/linux/spi/spi-offload.h
+K:	spi_offload
+
 SPI SUBSYSTEM
 M:	Mark Brown <broonie@kernel.org>
 L:	linux-spi@vger.kernel.org

drivers/spi/Kconfig

@@ -55,6 +55,9 @@ config SPI_MEM
 	  This extension is meant to simplify interaction with SPI memories
 	  by providing a high-level interface to send memory-like commands.
 
+config SPI_OFFLOAD
+	bool
+
 comment "SPI Master Controller Drivers"
 
 config SPI_AIROHA_SNFI

drivers/spi/Makefile

@@ -10,6 +10,7 @@ ccflags-$(CONFIG_SPI_DEBUG) := -DDEBUG
 obj-$(CONFIG_SPI_MASTER)		+= spi.o
 obj-$(CONFIG_SPI_MEM)			+= spi-mem.o
 obj-$(CONFIG_SPI_MUX)			+= spi-mux.o
+obj-$(CONFIG_SPI_OFFLOAD)		+= spi-offload.o
 obj-$(CONFIG_SPI_SPIDEV)		+= spidev.o
 obj-$(CONFIG_SPI_LOOPBACK_TEST)		+= spi-loopback-test.o
 

drivers/spi/spi-offload.c

@@ -0,0 +1,114 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2024 Analog Devices Inc.
+ * Copyright (C) 2024 BayLibre, SAS
+ */
+
+/*
+ * SPI Offloading support.
+ *
+ * Some SPI controllers support offloading of SPI transfers. Essentially, this
+ * is the ability for a SPI controller to perform SPI transfers with minimal
+ * or even no CPU intervention, e.g. via a specialized SPI controller with a
+ * hardware trigger or via a conventional SPI controller using a non-Linux MCU
+ * processor core to offload the work.
+ */
+
+#define DEFAULT_SYMBOL_NAMESPACE "SPI_OFFLOAD"
+
+#include <linux/cleanup.h>
+#include <linux/device.h>
+#include <linux/export.h>
+#include <linux/mutex.h>
+#include <linux/spi/offload/consumer.h>
+#include <linux/spi/offload/provider.h>
+#include <linux/spi/offload/types.h>
+#include <linux/spi/spi.h>
+#include <linux/types.h>
+
+struct spi_controller_and_offload {
+	struct spi_controller *controller;
+	struct spi_offload *offload;
+};
+
+/**
+ * devm_spi_offload_alloc() - Allocate offload instance
+ * @dev: Device for devm purposes and assigned to &struct spi_offload.provider_dev
+ * @priv_size: Size of private data to allocate
+ *
+ * Offload providers should use this to allocate offload instances.
+ *
+ * Return: Pointer to new offload instance or error on failure.
+ */
+struct spi_offload *devm_spi_offload_alloc(struct device *dev,
+					   size_t priv_size)
+{
+	struct spi_offload *offload;
+	void *priv;
+
+	offload = devm_kzalloc(dev, sizeof(*offload), GFP_KERNEL);
+	if (!offload)
+		return ERR_PTR(-ENOMEM);
+
+	priv = devm_kzalloc(dev, priv_size, GFP_KERNEL);
+	if (!priv)
+		return ERR_PTR(-ENOMEM);
+
+	offload->provider_dev = dev;
+	offload->priv = priv;
+
+	return offload;
+}
+EXPORT_SYMBOL_GPL(devm_spi_offload_alloc);
+
+static void spi_offload_put(void *data)
+{
+	struct spi_controller_and_offload *resource = data;
+
+	resource->controller->put_offload(resource->offload);
+	kfree(resource);
+}
+
+/**
+ * devm_spi_offload_get() - Get an offload instance
+ * @dev: Device for devm purposes
+ * @spi: SPI device to use for the transfers
+ * @config: Offload configuration
+ *
+ * Peripheral drivers call this function to get an offload instance that meets
+ * the requirements specified in @config. If no suitable offload instance is
+ * available, -ENODEV is returned.
+ *
+ * Return: Offload instance or error on failure.
+ */
+struct spi_offload *devm_spi_offload_get(struct device *dev,
+					 struct spi_device *spi,
+					 const struct spi_offload_config *config)
+{
+	struct spi_controller_and_offload *resource;
+	int ret;
+
+	if (!spi || !config)
+		return ERR_PTR(-EINVAL);
+
+	if (!spi->controller->get_offload)
+		return ERR_PTR(-ENODEV);
+
+	resource = kzalloc(sizeof(*resource), GFP_KERNEL);
+	if (!resource)
+		return ERR_PTR(-ENOMEM);
+
+	resource->controller = spi->controller;
+	resource->offload = spi->controller->get_offload(spi, config);
+	if (IS_ERR(resource->offload)) {
+		kfree(resource);
+		return resource->offload;
+	}
+
+	ret = devm_add_action_or_reset(dev, spi_offload_put, resource);
+	if (ret)
+		return ERR_PTR(ret);
+
+	return resource->offload;
+}
+EXPORT_SYMBOL_GPL(devm_spi_offload_get);

include/linux/spi/offload/consumer.h

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2024 Analog Devices Inc.
+ * Copyright (C) 2024 BayLibre, SAS
+ */
+
+#ifndef __LINUX_SPI_OFFLOAD_CONSUMER_H
+#define __LINUX_SPI_OFFLOAD_CONSUMER_H
+
+#include <linux/module.h>
+#include <linux/spi/offload/types.h>
+#include <linux/types.h>
+
+MODULE_IMPORT_NS("SPI_OFFLOAD");
+
+struct device;
+struct spi_device;
+
+struct spi_offload *devm_spi_offload_get(struct device *dev, struct spi_device *spi,
+					 const struct spi_offload_config *config);
+
+#endif /* __LINUX_SPI_OFFLOAD_CONSUMER_H */

include/linux/spi/offload/provider.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2024 Analog Devices Inc.
+ * Copyright (C) 2024 BayLibre, SAS
+ */
+
+#ifndef __LINUX_SPI_OFFLOAD_PROVIDER_H
+#define __LINUX_SPI_OFFLOAD_PROVIDER_H
+
+#include <linux/module.h>
+#include <linux/types.h>
+
+MODULE_IMPORT_NS("SPI_OFFLOAD");
+
+struct device;
+
+struct spi_offload *devm_spi_offload_alloc(struct device *dev, size_t priv_size);
+
+#endif /* __LINUX_SPI_OFFLOAD_PROVIDER_H */

include/linux/spi/offload/types.h

@@ -0,0 +1,43 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2024 Analog Devices Inc.
+ * Copyright (C) 2024 BayLibre, SAS
+ */
+
+#ifndef __LINUX_SPI_OFFLOAD_TYPES_H
+#define __LINUX_SPI_OFFLOAD_TYPES_H
+
+#include <linux/types.h>
+
+struct device;
+
+/* Offload can be triggered by external hardware event. */
+#define SPI_OFFLOAD_CAP_TRIGGER			BIT(0)
+/* Offload can record and then play back TX data when triggered. */
+#define SPI_OFFLOAD_CAP_TX_STATIC_DATA		BIT(1)
+/* Offload can get TX data from an external stream source. */
+#define SPI_OFFLOAD_CAP_TX_STREAM_DMA		BIT(2)
+/* Offload can send RX data to an external stream sink. */
+#define SPI_OFFLOAD_CAP_RX_STREAM_DMA		BIT(3)
+
+/**
+ * struct spi_offload_config - offload configuration
+ *
+ * This is used to request an offload with specific configuration.
+ */
+struct spi_offload_config {
+	/** @capability_flags: required capabilities. See %SPI_OFFLOAD_CAP_* */
+	u32 capability_flags;
+};
+
+/**
+ * struct spi_offload - offload instance
+ */
+struct spi_offload {
+	/** @provider_dev: for get/put reference counting */
+	struct device *provider_dev;
+	/** @priv: provider driver private data */
+	void *priv;
+};
+
+#endif /* __LINUX_SPI_OFFLOAD_TYPES_H */

include/linux/spi/spi.h

@@ -31,6 +31,8 @@ struct spi_transfer;
 struct spi_controller_mem_ops;
 struct spi_controller_mem_caps;
 struct spi_message;
+struct spi_offload;
+struct spi_offload_config;
 
 /*
  * INTERFACES between SPI master-side drivers and SPI slave protocol handlers,
@@ -496,6 +498,10 @@ extern struct spi_device *spi_new_ancillary_device(struct spi_device *spi, u8 ch
  * @mem_ops: optimized/dedicated operations for interactions with SPI memory.
  *	     This field is optional and should only be implemented if the
  *	     controller has native support for memory like operations.
+ * @get_offload: callback for controllers with offload support to get matching
+ *	offload instance. Implementations should return -ENODEV if no match is
+ *	found.
+ * @put_offload: release the offload instance acquired by @get_offload.
  * @mem_caps: controller capabilities for the handling of memory operations.
  * @unprepare_message: undo any work done by prepare_message().
  * @target_abort: abort the ongoing transfer request on an SPI target controller
@@ -740,6 +746,10 @@ struct spi_controller {
 	const struct spi_controller_mem_ops *mem_ops;
 	const struct spi_controller_mem_caps *mem_caps;
 
+	struct spi_offload *(*get_offload)(struct spi_device *spi,
+					   const struct spi_offload_config *config);
+	void (*put_offload)(struct spi_offload *offload);
+
 	/* GPIO chip select */
 	struct gpio_desc	**cs_gpiods;
 	bool			use_gpio_descriptors;
@@ -1108,6 +1118,7 @@ struct spi_transfer {
  * @state: for use by whichever driver currently owns the message
  * @opt_state: for use by whichever driver currently owns the message
  * @resources: for resource management when the SPI message is processed
+ * @offload: (optional) offload instance used by this message
  *
  * A @spi_message is used to execute an atomic sequence of data transfers,
  * each represented by a struct spi_transfer.  The sequence is "atomic"
@@ -1168,6 +1179,12 @@ struct spi_message {
 	 */
 	void			*opt_state;
 
+	/*
+	 * Optional offload instance used by this message. This must be set
+	 * by the peripheral driver before calling spi_optimize_message().
+	 */
+	struct spi_offload	*offload;
+
 	/* List of spi_res resources when the SPI message is processed */
 	struct list_head        resources;
 };