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

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Message-ID: <69cb2d42-6672-4c42-935f-e3fff9bf38f8@oss.qualcomm.com>
Date: Thu, 29 Jan 2026 16:09:41 +0530
From: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....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 1/6/2026 8:21 AM, Dmitry Baryshkov wrote:
> 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?
That's correct, GEM-PRIME will handle it properly. Here, the reason to add this
change is to enable routing of DSP logs to HLOS which is done by using a shared
buffer between userspace process and DSP PD. The buffer can be allocated from
both fastrpc driver's DMA-BUF or DMABUF heap(eg. system heap).

So this shared buffer is getting mapped to both process's IOMMU device and DSP PD
with this change.
>
>> 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?
I'll drop this.
>
>>  
>> -		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?
Ack. These should be part of uABI. I'll create a separate patch for this.
>
>> +		err = fastrpc_req_buf_alloc(fl, req, argp);
>> +		if (err)
>> +			return err;