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

[Vinayak Katoch <vinayak.katoch@oss.qualcomm.com>]

On 7/2/2026 11:05 AM, Ekansh Gupta wrote:
> On 09-06-2026 16:03, 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>;
>>
>> 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.
>>
>> 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;
> int i was already declared outside, avoid shadowing> +
Ack, will update in v2.>> +		spin_lock_irqsave(&data->lock, flags);
>> +		last_sess = &data->session[data->sesscount - 1];
>> +		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,
>