Re: [PATCH 3/3] misc: fastrpc: Use context device bus for compute banks

[Dmitry Baryshkov]

On Tue, Apr 14, 2026 at 10:01:17PM +0530, Ekansh Gupta via B4 Relay wrote:
> From: Ekansh Gupta <ekansh.gupta@xxxxxxxxxxxxxxxx>
> 
> Replace the platform driver approach for compute bank (CB) devices
> with the generic context_device_bus_type. Compute bank devices are
> synthetic IOMMU context banks, not real platform devices, so using
> the context device bus provides a more accurate representation in
> the device model.
> 
> Currently, fastrpc used of_platform_populate() to create platform
> devices for each "qcom,fastrpc-compute-cb" DT node, with a platform
> driver (fastrpc_cb_driver) to handle probe/remove. This approach
> had a race condition: device nodes were created before channel
> resources (like spin_lock) were initialized, and probe was async,
> so applications could open the device before sessions were available.
> 
> This patch addresses the race by manually creating and configuring
> CB devices synchronously during fastrpc_rpmsg_probe(), after all
> channel resources are initialized. The approach follows the pattern
> used in host1x_memory_context_list_init().
> 
> Signed-off-by: Ekansh Gupta <ekansh.gupta@xxxxxxxxxxxxxxxx>
> ---
>  drivers/misc/Kconfig   |   1 +
>  drivers/misc/fastrpc.c | 180 ++++++++++++++++++++++++++++++++++---------------
>  2 files changed, 125 insertions(+), 56 deletions(-)
> 
> diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
> index 00683bf06258..b501462a4548 100644
> --- a/drivers/misc/Kconfig
> +++ b/drivers/misc/Kconfig
> @@ -304,6 +304,7 @@ config QCOM_FASTRPC
>  	depends on RPMSG
>  	select DMA_SHARED_BUFFER
>  	select QCOM_SCM
> +	select CONTEXT_DEVICE_BUS
>  	help
>  	  Provides a communication mechanism that allows for clients to
>  	  make remote method invocations across processor boundary to
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 1080f9acf70a..f66fd3eea5fa 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -13,9 +13,9 @@
>  #include <linux/module.h>
>  #include <linux/of_address.h>
>  #include <linux/of.h>
> -#include <linux/platform_device.h>
> +#include <linux/of_device.h>
>  #include <linux/sort.h>
> -#include <linux/of_platform.h>
> +#include <linux/context_bus.h>
>  #include <linux/rpmsg.h>
>  #include <linux/scatterlist.h>
>  #include <linux/slab.h>
> @@ -250,6 +250,18 @@ struct fastrpc_invoke_ctx {
>  	struct fastrpc_channel_ctx *cctx;
>  };
>  
> +/**
> + * struct fastrpc_cb_device - Compute bank device wrapper
> + * @dev: Device structure
> + * @sess: Back-pointer to the session context
> + */
> +struct fastrpc_cb_device {
> +	struct device dev;
> +	struct fastrpc_session_ctx *sess;
> +};
> +
> +#define to_fastrpc_cb_device(d) container_of(d, struct fastrpc_cb_device, dev)
> +
>  struct fastrpc_session_ctx {
>  	struct device *dev;
>  	int sid;
> @@ -2190,92 +2202,156 @@ static const struct file_operations fastrpc_fops = {
>  	.compat_ioctl = fastrpc_device_ioctl,
>  };
>  
> -static int fastrpc_cb_probe(struct platform_device *pdev)
> +static void fastrpc_cb_dev_release(struct device *dev)
> +{
> +	struct fastrpc_cb_device *cb_dev = to_fastrpc_cb_device(dev);
> +
> +	of_node_put(dev->of_node);
> +	kfree(cb_dev);
> +}
> +
> +static int fastrpc_create_cb_device(struct fastrpc_channel_ctx *cctx,
> +				    struct device *parent,
> +				    struct device_node *cb_node)
>  {
> -	struct fastrpc_channel_ctx *cctx;
>  	struct fastrpc_session_ctx *sess;
> -	struct device *dev = &pdev->dev;
> -	int i, sessions = 0;
> +	struct fastrpc_cb_device *cb_dev;
>  	unsigned long flags;
> -	int rc;
> -	u32 dma_bits;
> -
> -	cctx = dev_get_drvdata(dev->parent);
> -	if (!cctx)
> -		return -EINVAL;
> +	int i, sessions = 0, rc;
> +	u32 dma_bits, sid = 0;
>  
> -	of_property_read_u32(dev->of_node, "qcom,nsessions", &sessions);
> +	/* Read SID early so it can be used in the device name */
> +	of_property_read_u32(cb_node, "reg", &sid);
> +	of_property_read_u32(cb_node, "qcom,nsessions", &sessions);
>  
>  	spin_lock_irqsave(&cctx->lock, flags);
>  	if (cctx->sesscount >= FASTRPC_MAX_SESSIONS) {
> -		dev_err(&pdev->dev, "too many sessions\n");
> +		dev_err(parent, "too many sessions\n");
>  		spin_unlock_irqrestore(&cctx->lock, flags);
>  		return -ENOSPC;
>  	}
>  	dma_bits = cctx->soc_data->dma_addr_bits_default;
> +	if (cctx->domain_id == CDSP_DOMAIN_ID)
> +		dma_bits = cctx->soc_data->dma_addr_bits_cdsp;
> +
>  	sess = &cctx->session[cctx->sesscount++];
>  	sess->used = false;
>  	sess->valid = true;
> -	sess->dev = dev;
> -	dev_set_drvdata(dev, sess);
> +	sess->sid = sid;
> +	spin_unlock_irqrestore(&cctx->lock, flags);
>  
> -	if (cctx->domain_id == CDSP_DOMAIN_ID)
> -		dma_bits = cctx->soc_data->dma_addr_bits_cdsp;
> +	cb_dev = kzalloc_obj(*cb_dev);
> +	if (!cb_dev)
> +		return -ENOMEM;
>  
> -	if (of_property_read_u32(dev->of_node, "reg", &sess->sid))
> -		dev_info(dev, "FastRPC Session ID not specified in DT\n");
> +	cb_dev->sess = sess;
>  
> -	if (sessions > 0) {
> -		struct fastrpc_session_ctx *dup_sess;
> +	device_initialize(&cb_dev->dev);
> +	cb_dev->dev.parent = parent;
> +	cb_dev->dev.bus = &context_device_bus_type;
> +	cb_dev->dev.release = fastrpc_cb_dev_release;
> +	cb_dev->dev.of_node = of_node_get(cb_node);
> +	cb_dev->dev.dma_mask = &cb_dev->dev.coherent_dma_mask;
> +	cb_dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
> +	dev_set_name(&cb_dev->dev, "%s:compute-cb@%u", dev_name(parent), sid);

Please extract this code to a helper function inside the context bus and
use it for both host1x and fastrpc.

>  
> +	rc = device_add(&cb_dev->dev);
> +	if (rc) {
> +		dev_err(parent, "failed to add CB device: %d\n", rc);
> +		goto err_put;
> +	}
> +
> +	rc = of_dma_configure(&cb_dev->dev, cb_node, true);
> +	if (rc) {
> +		dev_err(parent, "of_dma_configure failed for CB device: %d\n", rc);
> +		goto err_del;
> +	}
> +
> +	rc = dma_set_mask(&cb_dev->dev, DMA_BIT_MASK(dma_bits));
> +	if (rc) {
> +		dev_err(parent, "%u-bit DMA enable failed\n", dma_bits);
> +		goto err_del;
> +	}
> +
> +	sess->dev = &cb_dev->dev;
> +
> +	if (sessions > 0) {
> +		spin_lock_irqsave(&cctx->lock, flags);
>  		for (i = 1; i < sessions; i++) {
> +			struct fastrpc_session_ctx *dup_sess;
> +
>  			if (cctx->sesscount >= FASTRPC_MAX_SESSIONS)
>  				break;
>  			dup_sess = &cctx->session[cctx->sesscount++];
>  			memcpy(dup_sess, sess, sizeof(*dup_sess));
>  		}
> -	}
> -	spin_unlock_irqrestore(&cctx->lock, flags);
> -	rc = dma_set_mask(dev, DMA_BIT_MASK(dma_bits));
> -	if (rc) {
> -		dev_err(dev, "%u-bit DMA enable failed\n", dma_bits);
> -		return rc;
> +		spin_unlock_irqrestore(&cctx->lock, flags);
>  	}
>  
>  	return 0;
> +
> +err_del:
> +	device_del(&cb_dev->dev);
> +err_put:
> +	of_node_put(cb_dev->dev.of_node);
> +	put_device(&cb_dev->dev);
> +	return rc;
>  }
>  
> -static void fastrpc_cb_remove(struct platform_device *pdev)
> +static void fastrpc_depopulate_cb_devices(struct fastrpc_channel_ctx *cctx)
>  {
> -	struct fastrpc_channel_ctx *cctx = dev_get_drvdata(pdev->dev.parent);
> -	struct fastrpc_session_ctx *sess = dev_get_drvdata(&pdev->dev);
>  	unsigned long flags;
> -	int i;
> +	int i, j;
>  
>  	spin_lock_irqsave(&cctx->lock, flags);
>  	for (i = 0; i < FASTRPC_MAX_SESSIONS; i++) {
> -		if (cctx->session[i].sid == sess->sid) {
> +		if (cctx->session[i].valid) {
>  			cctx->session[i].valid = false;
>  			cctx->sesscount--;
>  		}
>  	}
>  	spin_unlock_irqrestore(&cctx->lock, flags);
> +
> +	for (i = 0; i < FASTRPC_MAX_SESSIONS; i++) {
> +		struct device *dev = cctx->session[i].dev;
> +
> +		if (!dev)
> +			continue;
> +
> +		/* Unregister the device once */
> +		device_unregister(dev);
> +
> +		/* Clear this dev pointer from all sessions that share it */
> +		for (j = i; j < FASTRPC_MAX_SESSIONS; j++) {
> +			if (cctx->session[j].dev == dev)
> +				cctx->session[j].dev = NULL;
> +		}
> +	}
>  }
>  
> -static const struct of_device_id fastrpc_match_table[] = {
> -	{ .compatible = "qcom,fastrpc-compute-cb", },
> -	{}
> -};
> +static int fastrpc_populate_cb_devices(struct fastrpc_channel_ctx *cctx,
> +					struct device *parent,
> +					struct device_node *parent_node)
> +{
> +	struct device_node *child;
> +	int ret = 0;
>  
> -static struct platform_driver fastrpc_cb_driver = {
> -	.probe = fastrpc_cb_probe,
> -	.remove = fastrpc_cb_remove,
> -	.driver = {
> -		.name = "qcom,fastrpc-cb",
> -		.of_match_table = fastrpc_match_table,
> -		.suppress_bind_attrs = true,
> -	},
> -};
> +	for_each_child_of_node(parent_node, child) {
> +		if (!of_device_is_compatible(child, "qcom,fastrpc-compute-cb"))
> +			continue;
> +
> +		ret = fastrpc_create_cb_device(cctx, parent, child);
> +		if (ret) {
> +			dev_err(parent, "failed to create CB device for %s: %d\n",
> +				child->name, ret);
> +			of_node_put(child);
> +			fastrpc_depopulate_cb_devices(cctx);
> +			return ret;
> +		}
> +	}
> +
> +	return 0;
> +}
>  
>  static int fastrpc_device_register(struct device *dev, struct fastrpc_channel_ctx *cctx,
>  				   bool is_secured, const char *domain)
> @@ -2441,7 +2517,7 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  	data->domain_id = domain_id;
>  	data->rpdev = rpdev;
>  
> -	err = of_platform_populate(rdev->of_node, NULL, NULL, rdev);
> +	err = fastrpc_populate_cb_devices(data, rdev, rdev->of_node);
>  	if (err)
>  		goto err_deregister_fdev;
>  
> @@ -2496,7 +2572,7 @@ static void fastrpc_rpmsg_remove(struct rpmsg_device *rpdev)
>  	if (cctx->remote_heap)
>  		fastrpc_buf_free(cctx->remote_heap);
>  
> -	of_platform_depopulate(&rpdev->dev);
> +	fastrpc_depopulate_cb_devices(cctx);
>  
>  	fastrpc_channel_ctx_put(cctx);
>  }
> @@ -2558,16 +2634,9 @@ static int fastrpc_init(void)
>  {
>  	int ret;
>  
> -	ret = platform_driver_register(&fastrpc_cb_driver);
> -	if (ret < 0) {
> -		pr_err("fastrpc: failed to register cb driver\n");
> -		return ret;
> -	}
> -
>  	ret = register_rpmsg_driver(&fastrpc_driver);
>  	if (ret < 0) {
>  		pr_err("fastrpc: failed to register rpmsg driver\n");
> -		platform_driver_unregister(&fastrpc_cb_driver);
>  		return ret;
>  	}
>  
> @@ -2577,7 +2646,6 @@ module_init(fastrpc_init);
>  
>  static void fastrpc_exit(void)
>  {
> -	platform_driver_unregister(&fastrpc_cb_driver);
>  	unregister_rpmsg_driver(&fastrpc_driver);
>  }
>  module_exit(fastrpc_exit);
> 
> -- 
> 2.34.1
> 
> 

-- 
With best wishes
Dmitry