Re: [PATCH] misc: fastrpc: create duplicate sessions after all CB probing

[Srinivas Kandagatla <srini@kernel.org>]

On 6/9/26 11:33 AM, Vinayak Katoch wrote:
> For ADSP, only a limited number of FastRPC context banks (CBs) are
> available. Each CB supports a single session, which means only a few
> processes can run on ADSP simultaneously. If all sessions are consumed
> by fastrpc daemons, no session remains available when a user application
> starts, causing the application to fail.
> 
> To address this limitation, a Device Tree change was used till now:
>   qcom,nsessions = <5>;
> 
You should mark this property as deprecated in dt bindings.
Which should discourage people to use this property.


> However, feedback from the upstream community indicated that this change
> should not be made in the Device Tree. Instead, it was recommended to
> handle this as a driver-level change.

Changing it in driver will make it applicable for all the SoCs.

> 
> Instead of duplicating sessions inline during fastrpc_cb_probe() using
> the qcom,nsessions DT property, defer duplication until after
> of_platform_populate() returns in fastrpc_rpmsg_probe(), at which point
> all compute-CB child nodes have been probed and the session array is
> fully populated.
> 
> For the ADSP domain, append FASTRPC_DUP_SESSIONS (4) copies of the
> last probed session once of_platform_populate() succeeds. This keeps
> the per-CB probe path simple and ensures duplicates are always derived
> from a stable, fully-initialised session state.
> 
> The qcom,nsessions DT property is no longer consumed by the driver; the
> binding and DT sources are left unchanged.
> 
> Signed-off-by: Vinayak Katoch <vinayak.katoch@oss.qualcomm.com>
> ---
>  drivers/misc/fastrpc.c | 31 ++++++++++++++++++-------------
>  1 file changed, 18 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 1080f9acf70a..46afbae9c234 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -30,6 +30,7 @@
>  #define CDSP_DOMAIN_ID (3)
>  #define GDSP_DOMAIN_ID (4)
>  #define FASTRPC_MAX_SESSIONS	14
> +#define FASTRPC_DUP_SESSIONS	4
>  #define FASTRPC_MAX_VMIDS	16
>  #define FASTRPC_ALIGN		128
>  #define FASTRPC_MAX_FDLIST	16
> @@ -2195,7 +2196,6 @@ static int fastrpc_cb_probe(struct platform_device *pdev)
>  	struct fastrpc_channel_ctx *cctx;
>  	struct fastrpc_session_ctx *sess;
>  	struct device *dev = &pdev->dev;
> -	int i, sessions = 0;
>  	unsigned long flags;
>  	int rc;
>  	u32 dma_bits;
> @@ -2204,8 +2204,6 @@ static int fastrpc_cb_probe(struct platform_device *pdev)
>  	if (!cctx)
>  		return -EINVAL;
>  
> -	of_property_read_u32(dev->of_node, "qcom,nsessions", &sessions);
> -
>  	spin_lock_irqsave(&cctx->lock, flags);
>  	if (cctx->sesscount >= FASTRPC_MAX_SESSIONS) {
>  		dev_err(&pdev->dev, "too many sessions\n");
> @@ -2225,16 +2223,6 @@ static int fastrpc_cb_probe(struct platform_device *pdev)
>  	if (of_property_read_u32(dev->of_node, "reg", &sess->sid))
>  		dev_info(dev, "FastRPC Session ID not specified in DT\n");
>  
> -	if (sessions > 0) {
> -		struct fastrpc_session_ctx *dup_sess;
> -
> -		for (i = 1; i < sessions; i++) {
> -			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) {
> @@ -2445,6 +2433,23 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  	if (err)
>  		goto err_deregister_fdev;
>  
> +	if (data->domain_id == ADSP_DOMAIN_ID && data->sesscount > 0) {
> +		struct fastrpc_session_ctx *last_sess;
> +		struct fastrpc_session_ctx *dup_sess;
> +		unsigned long flags;
> +		int i;
> +
> +		spin_lock_irqsave(&data->lock, flags);
> +		last_sess = &data->session[data->sesscount - 1];

Why only for last session?

This is now un conditionally done for ADSP which changes the whole
behaviour.

> +		for (i = 0; i < FASTRPC_DUP_SESSIONS; i++) {
> +			if (data->sesscount >= FASTRPC_MAX_SESSIONS)
> +				break;
> +			dup_sess = &data->session[data->sesscount++];
> +			memcpy(dup_sess, last_sess, sizeof(*dup_sess));
> +		}
> +		spin_unlock_irqrestore(&data->lock, flags);
> +	}
> +
>  	return 0;
>  
>  err_deregister_fdev:
> 
> ---
> base-commit: 97e797263a5e963da3d1e66e743fd518567dfe37
> change-id: 20260609-dup-sessions-ea2acaac1994
> 
> Best regards,