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

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Message-ID: <k2jiy66ruyfh4v5go346e7wa2y6ujxjzl543imq3n3k76z3dju@jvjwm2k543k7>
Date: Mon, 9 Feb 2026 03:39:54 +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 Fri, Jan 30, 2026 at 05:27:37PM +0530, Ekansh Gupta wrote:
> 
> 
> 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.

I was thinking about dma_buf_attach() rather than
dma_buf_map_attachment().

> 
> 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@....qualcomm.com>
> >>>> ---
> >>>>  drivers/misc/fastrpc.c | 97 +++++++++++++++++++++++++++++++++++++-----
> >>>>  1 file changed, 86 insertions(+), 11 deletions(-)
> >>>>
> 

-- 
With best wishes
Dmitry