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

[Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>]

On 1/30/2026 7:59 AM, Dmitry Baryshkov wrote:
> On Thu, Jan 29, 2026 at 04:09:41PM +0530, Ekansh Gupta wrote:
>>
>> 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.
> So, you have the DMA-BUF buffer. Instead of mapping it from userspace
> with unclean semantics, please import the buffer.
I'm assuming fastrpc_map_create is sort of importing the buffer by calling
dma_buf_map_attachment. Is this not the correct understanding? This assumption
is based on the existing part inside fastrpc_get_args, where fastrpc_map_create is
getting called for each of user passed DMA-BUF.

Moving to accel based driver is going to standardize this as the .gem_prime_import
implementation is going to handle this case.

//Ekansh

>
>>>> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>>>> ---
>>>>  drivers/misc/fastrpc.c | 97 +++++++++++++++++++++++++++++++++++++-----
>>>>  1 file changed, 86 insertions(+), 11 deletions(-)
>>>>