linux-kernel - Re: [PATCH v3 3/3] misc: fastrpc: Support mapping userspace-allocated buffers

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Message-ID: <z3tktit6jkxrkre4gm666aw3ql3plyhs6266cu2itrbjbj2das@yjp2pyesoszn>
Date: Tue, 6 Jan 2026 04:51:18 +0200
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 v3 3/3] misc: fastrpc: Support mapping
 userspace-allocated buffers

On Tue, Dec 30, 2025 at 04:32:25PM +0530, Ekansh Gupta wrote:
> Currently, FastRPC only supports mapping buffers allocated by the
> kernel. This limits flexibility for applications that allocate memory
> in userspace using rpcmem or DMABUF and need to share it with the DSP.

Hmm, for DMABUF we need _import_ support rather than support for mapping
of userspace-allocated buffers.

> Add support for mapping and unmapping userspace-allocated buffers to
> the DSP through SMMU. This includes handling map requests for rpcmem
> and DMABUF-backed memory and providing corresponding unmap
> functionality.

For me this definitely looks like a step back. For drm/accel we are
going to have GEM-managed buffers only. Why do we need to handle
userspace-allocated buffers here?

> 
> Signed-off-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
> ---
>  drivers/misc/fastrpc.c | 97 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 86 insertions(+), 11 deletions(-)
> 
> @@ -1989,25 +2020,69 @@ static int fastrpc_req_buf_alloc(struct fastrpc_user *fl,
>  	return err;
>  }
>  
> -static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
> +static int fastrpc_req_map_create(struct fastrpc_user *fl,
> +				  struct fastrpc_req_mmap req,
> +				  char __user *argp)
>  {
> -	struct fastrpc_req_mmap req;
> +	struct fastrpc_map *map = NULL;
> +	struct device *dev = fl->sctx->dev;
> +	u64 raddr = 0;
>  	int err;
>  
> -	if (copy_from_user(&req, argp, sizeof(req)))
> -		return -EFAULT;
> +	err = fastrpc_map_create(fl, req.fd, req.size, 0, &map);
> +	if (err) {
> +		dev_err(dev, "failed to map buffer, fd = %d\n", req.fd);
> +		return err;
> +	}
> +
> +	err = fastrpc_req_map_dsp(fl, map->phys, map->size, req.flags,
> +				  req.vaddrin, &raddr);
> +	if (err)
> +		goto err_invoke;
>  
> -	if (req.flags != ADSP_MMAP_ADD_PAGES && req.flags != ADSP_MMAP_REMOTE_HEAP_ADDR) {
> -		dev_err(fl->sctx->dev, "flag not supported 0x%x\n", req.flags);
> +	/* update the buffer to be able to deallocate the memory on the DSP */
> +	map->raddr = (u64)raddr;

Which type are you converting? And why?

>  
> -		return -EINVAL;
> +	/* let the client know the address to use */
> +	req.vaddrout = raddr;
> +	dev_dbg(dev, "mmap OK: raddr=%p [len=0x%08llx]\n",
> +		(void *)(unsigned long)map->raddr, map->size);
> +
> +	if (copy_to_user(argp, &req, sizeof(req))) {
> +		err = -EFAULT;
> +		goto err_copy;
>  	}
>  
> -	err = fastrpc_req_buf_alloc(fl, req, argp);
> +	return 0;
> +err_copy:
> +	fastrpc_req_munmap_dsp(fl, map->raddr, map->size);
> +err_invoke:
> +	fastrpc_map_put(map);
>  
>  	return err;
>  }
>  
> +static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
> +{
> +	struct fastrpc_req_mmap req;
> +	int err;
> +
> +	if (copy_from_user(&req, argp, sizeof(req)))
> +		return -EFAULT;
> +
> +	if ((req.flags == ADSP_MMAP_ADD_PAGES ||
> +	     req.flags == ADSP_MMAP_REMOTE_HEAP_ADDR)) {

Side note: why are these flags not defined in the uABI header?

> +		err = fastrpc_req_buf_alloc(fl, req, argp);
> +		if (err)
> +			return err;

-- 
With best wishes
Dmitry