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

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Message-ID: <33xoq7nt7j3sgcrnxjmbfogrdy3hyskym3yzurmqzwdqxtlgwe@vywpjmegteel>
Date: Fri, 30 Jan 2026 04:29:10 +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 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.

> >
> >> 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