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Message-ID: <hqbazo62hdfwgxoevzkchfddvjpr2ttp7wltpkoooou5anongs@5ncpjec3egjh>
Date: Tue, 2 Sep 2025 12:21:19 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
Cc: srini@...nel.org, linux-arm-msm@...r.kernel.org,
        gregkh@...uxfoundation.org, quic_bkumar@...cinc.com,
        linux-kernel@...r.kernel.org, quic_chennak@...cinc.com,
        dri-devel@...ts.freedesktop.org, arnd@...db.de
Subject: Re: [PATCH v1 3/3] misc: fastrpc: Add polling mode support for
 fastRPC driver

On Mon, Sep 01, 2025 at 11:03:36AM +0530, Ekansh Gupta wrote:
> For any remote call to DSP, after sending an invocation message,
> fastRPC driver waits for glink response and during this time the
> CPU can go into low power modes. This adds latency to overall fastrpc
> call as CPU wakeup and scheduling latencies are included.  Adding a

s/Adding/Add/, see Documentation/process/submitting-patches.rst

> polling mode support with which fastRPC driver will poll continuously
> on a memory after sending a message to remote subsystem which will
> eliminate CPU wakeup and scheduling latencies and reduce fastRPC
> overhead.

Describe your design decisions: when it is enabled, why, etc.

> 
> Signed-off-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
> ---
>  drivers/misc/fastrpc.c | 121 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 114 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 57e118de6e4a..939a3e3d29e2 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -22,6 +22,8 @@
>  #include <linux/firmware/qcom/qcom_scm.h>
>  #include <uapi/misc/fastrpc.h>
>  #include <linux/of_reserved_mem.h>
> +#include <linux/compiler.h>
> +#include <linux/iopoll.h>
>  
>  #define ADSP_DOMAIN_ID (0)
>  #define MDSP_DOMAIN_ID (1)
> @@ -37,6 +39,7 @@
>  #define FASTRPC_CTX_MAX (256)
>  #define FASTRPC_INIT_HANDLE	1
>  #define FASTRPC_DSP_UTILITIES_HANDLE	2
> +#define FASTRPC_MAX_STATIC_HANDLE (20)

What is this?

>  #define FASTRPC_CTXID_MASK (0xFF00)
>  #define INIT_FILELEN_MAX (2 * 1024 * 1024)
>  #define INIT_FILE_NAMELEN_MAX (128)
> @@ -105,6 +108,20 @@
>  
>  #define miscdev_to_fdevice(d) container_of(d, struct fastrpc_device, miscdev)
>  
> +/* Poll response number from remote processor for call completion */
> +#define FASTRPC_POLL_RESPONSE (0xdecaf)
> +
> +/* Polling mode timeout limit */
> +#define FASTRPC_POLL_MAX_TIMEOUT_US (10000)
> +
> +/* Response types supported for RPC calls */
> +enum fastrpc_response_flags {
> +	/* normal job completion glink response */
> +	NORMAL_RESPONSE = 0,
> +	/* process updates poll memory instead of glink response */
> +	POLL_MODE = 1,
> +};

bool is_polled;

OR

unsigned long is_polled : 1;

> +
>  struct fastrpc_phy_page {
>  	u64 addr;		/* physical address */
>  	u64 size;		/* size of contiguous region */
> @@ -235,8 +252,14 @@ struct fastrpc_invoke_ctx {
>  	u32 sc;
>  	u64 *fdlist;
>  	u32 *crc;
> +	/* Poll memory that DSP updates */
> +	u32 *poll;
>  	u64 ctxid;
>  	u64 msg_sz;
> +	/* work done status flag */
> +	bool is_work_done;
> +	/* response flags from remote processor */
> +	enum fastrpc_response_flags rsp_flags;
>  	struct kref refcount;
>  	struct list_head node; /* list of ctxs */
>  	struct completion work;
> @@ -891,7 +914,8 @@ static int fastrpc_get_meta_size(struct fastrpc_invoke_ctx *ctx)
>  		sizeof(struct fastrpc_invoke_buf) +
>  		sizeof(struct fastrpc_phy_page)) * ctx->nscalars +
>  		sizeof(u64) * FASTRPC_MAX_FDLIST +
> -		sizeof(u32) * FASTRPC_MAX_CRCLIST;
> +		sizeof(u32) * FASTRPC_MAX_CRCLIST +
> +		sizeof(u32);
>  
>  	return size;
>  }
> @@ -987,6 +1011,8 @@ static int fastrpc_get_args(u32 kernel, struct fastrpc_invoke_ctx *ctx)
>  	list = fastrpc_invoke_buf_start(rpra, ctx->nscalars);
>  	pages = fastrpc_phy_page_start(list, ctx->nscalars);
>  	ctx->fdlist = (u64 *)(pages + ctx->nscalars);
> +	ctx->crc = (u32 *)(ctx->fdlist + FASTRPC_MAX_FDLIST);

Why?

> +	ctx->poll = (u32 *)(ctx->crc + FASTRPC_MAX_CRCLIST);
>  	args = (uintptr_t)ctx->buf->virt + metalen;
>  	rlen = pkt_size - metalen;
>  	ctx->rpra = rpra;
> @@ -1155,6 +1181,83 @@ static int fastrpc_invoke_send(struct fastrpc_session_ctx *sctx,
>  
>  }
>  
> +static inline u32 fastrpc_poll_op(void *p)
> +{
> +	struct fastrpc_invoke_ctx *ctx = p;
> +
> +	dma_rmb();
> +	return READ_ONCE(*ctx->poll);

Is this enough? Is the write by the DSP side going to invalidate the
cache for this memory location? Think about older platforms which
usually don't have dma-coherent property in the DSP / FastRPC nodes.

> +}
> +
> +static int poll_for_remote_response(struct fastrpc_invoke_ctx *ctx)
> +{
> +	u32 val;
> +	int ret;
> +
> +	/*
> +	 * Poll until DSP writes FASTRPC_POLL_RESPONSE into *ctx->poll
> +	 * or until another path marks the work done.
> +	 */
> +	ret = read_poll_timeout_atomic(fastrpc_poll_op, val,
> +				       (val == FASTRPC_POLL_RESPONSE) ||
> +				       ctx->is_work_done, 1,
> +				       FASTRPC_POLL_MAX_TIMEOUT_US, false, ctx);
> +
> +	if (!ret && val == FASTRPC_POLL_RESPONSE) {
> +		ctx->is_work_done = true;
> +		ctx->retval = 0;
> +	}
> +
> +	if (ret == -ETIMEDOUT)
> +		ret = -EIO;
> +
> +	return ret;
> +}
> +
> +static inline int fastrpc_wait_for_response(struct fastrpc_invoke_ctx *ctx,
> +					    u32 kernel)
> +{
> +	int err = 0;
> +
> +	if (kernel) {
> +		if (!wait_for_completion_timeout(&ctx->work, 10 * HZ))
> +			err = -ETIMEDOUT;
> +	} else {
> +		err = wait_for_completion_interruptible(&ctx->work);
> +	}
> +
> +	return err;
> +}
> +
> +static int fastrpc_wait_for_completion(struct fastrpc_invoke_ctx *ctx,
> +				       u32 kernel)
> +{
> +	int err;
> +
> +	do {
> +		switch (ctx->rsp_flags) {
> +		case NORMAL_RESPONSE:
> +			err = fastrpc_wait_for_response(ctx, kernel);
> +			if (err || ctx->is_work_done)
> +				return err;
> +			break;
> +		case POLL_MODE:
> +			err = poll_for_remote_response(ctx);
> +			/* If polling timed out, move to normal response mode */
> +			if (err)
> +				ctx->rsp_flags = NORMAL_RESPONSE;
> +			break;
> +		default:

What kind of response type can it be? Have you had checked for the flag
being set, you wouldn't have a false possibility of having another
response type.

> +			err = -EBADR;
> +			dev_dbg(ctx->fl->sctx->dev,
> +				"unsupported response type:0x%x\n", ctx->rsp_flags);
> +			break;
> +		}
> +	} while (!ctx->is_work_done);
> +
> +	return err;
> +}
> +
>  static int fastrpc_internal_invoke(struct fastrpc_user *fl,  u32 kernel,
>  				   u32 handle, u32 sc,
>  				   struct fastrpc_invoke_args *args)
> @@ -1190,16 +1293,19 @@ static int fastrpc_internal_invoke(struct fastrpc_user *fl,  u32 kernel,
>  	if (err)
>  		goto bail;
>  
> -	if (kernel) {
> -		if (!wait_for_completion_timeout(&ctx->work, 10 * HZ))
> -			err = -ETIMEDOUT;
> -	} else {
> -		err = wait_for_completion_interruptible(&ctx->work);
> -	}
> +	if (handle > FASTRPC_MAX_STATIC_HANDLE && fl->pd == USER_PD)
> +		ctx->rsp_flags = POLL_MODE;

This definitely needs to be explained.

>  
> +	err = fastrpc_wait_for_completion(ctx, kernel);
>  	if (err)
>  		goto bail;
>  
> +	if (!ctx->is_work_done) {
> +		err = -ETIMEDOUT;
> +		dev_dbg(fl->sctx->dev, "Invalid workdone state for handle 0x%x, sc 0x%x\n",
> +			handle, sc);
> +		goto bail;
> +	}
>  	/* make sure that all memory writes by DSP are seen by CPU */
>  	dma_rmb();
>  	/* populate all the output buffers with results */
> @@ -2462,6 +2568,7 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
>  
>  	ctx->retval = rsp->retval;
>  	complete(&ctx->work);
> +	ctx->is_work_done = true;
>  
>  	/*
>  	 * The DMA buffer associated with the context cannot be freed in
> -- 
> 2.34.1
> 

-- 
With best wishes
Dmitry

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