Re: [PATCH v2 06/34] staging: vc04_services: Add new vc-sm-cma driver

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Dave,

On Wed, May 06, 2020 at 08:24:38PM +0100, Dave Stevenson wrote:
> On Wed, 6 May 2020 at 19:04, Nicolas Saenz Julienne wrote:
> > On Mon, 2020-05-04 at 12:25 +0300, Laurent Pinchart wrote:
> >> From: Dave Stevenson <dave.stevenson@raspberrypi.org>
> >>
> >> Add Broadcom VideoCore Shared Memory support.
> >>
> >> This new driver allows contiguous memory blocks to be imported
> >> into the VideoCore VPU memory map, and manages the lifetime of
> >> those objects, only releasing the source dmabuf once the VPU has
> >> confirmed it has finished with it.
> >
> > I'm still digesting all this, but a question came up, who is using the
> > ioctls?
> 
> We have a userspace library that uses it [1].
> It is used by things like MMAL to share buffers between the VPU and
> ARM, rather than having to get VCHI to copy all the data between
> mirrored buffers.
> 
> I think what has happened here is that Laurent has picked up the
> version of the driver from the top of our downstream kernel tree.
> For libcamera and the ISP driver, we need a significantly smaller
> feature set, basically import of dmabufs only, no allocations or cache
> management. For the ISP driver it's mainly dmabuf import from
> videobuf2 for the image buffers, but there's also a need to pass in
> lens shading tables which are relatively large. With a small amount of
> rework in libcamera, we can make it so that we use dma-buf heaps to do
> the allocation, and pass in a dmabuf fd to the ISP driver to then map
> onto the VPU. That removes all the ioctls handling from this driver.
> 
> Downstream we do have other use cases that want to be able to do other
> functions on shared memory, but that too should be reworkable into
> using dma-buf heaps for allocations, and vcsm only handles importing
> dmabufs via an ioctl. All that can be hidden away in the vcsm library,
> so applications don't care.
> We've also got some legacy code kicking around, as there was
> originally a version of the driver that mapped the VPU's memory blocks
> to the ARM. That's why the vcsm library has two code paths through
> almost every function - one for each driver.
> 
> Laurent: What's your view? Halt the review this particular patch for
> now and rework, or try and get this all integrated?
> Mainline obviously already has dma-buf heaps merged, whilst I have a
> PR cherry-picking it back into our downstream 5.4. The main reason it
> hasn't been merged is that I haven't had a test case to prove it
> works. The rework should be relatively simple, but will need small
> updates to both libcamera and ISP driver.

My preference would be to go for a solution based on dma-buf heap right
away for mainline, to minimize the code going into staging. There's no
hurry there, and I can help integrating the changes in libcamera if
needed.

> [1] https://github.com/raspberrypi/userland/tree/master/host_applications/linux/libs/sm
> 
> >> Driver upported from the RaspberryPi BSP at revision:
> >> 890691d1c996 ("staging: vc04_services: Fix vcsm overflow bug when
> >> counting transactions")
> >> forward ported to recent mainline kernel version.
> >>
> >> Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
> >> Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
> >> Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
> >> ---
> >>  drivers/staging/vc04_services/Kconfig         |    2 +
> >>  drivers/staging/vc04_services/Makefile        |    1 +
> >>  .../include/linux/broadcom/vc_sm_cma_ioctl.h  |  114 ++
> >>  .../staging/vc04_services/vc-sm-cma/Kconfig   |   10 +
> >>  .../staging/vc04_services/vc-sm-cma/Makefile  |   13 +
> >>  drivers/staging/vc04_services/vc-sm-cma/TODO  |    1 +
> >>  .../staging/vc04_services/vc-sm-cma/vc_sm.c   | 1732 +++++++++++++++++
> >>  .../staging/vc04_services/vc-sm-cma/vc_sm.h   |   84 +
> >>  .../vc04_services/vc-sm-cma/vc_sm_cma_vchi.c  |  505 +++++
> >>  .../vc04_services/vc-sm-cma/vc_sm_cma_vchi.h  |   63 +
> >>  .../vc04_services/vc-sm-cma/vc_sm_defs.h      |  300 +++
> >>  .../vc04_services/vc-sm-cma/vc_sm_knl.h       |   28 +
> >>  12 files changed, 2853 insertions(+)
> >>  create mode 100644
> >> drivers/staging/vc04_services/include/linux/broadcom/vc_sm_cma_ioctl.h
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/Kconfig
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/Makefile
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/TODO
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/vc_sm.c
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/vc_sm.h
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/vc_sm_cma_vchi.c
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/vc_sm_cma_vchi.h
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/vc_sm_defs.h
> >>  create mode 100644 drivers/staging/vc04_services/vc-sm-cma/vc_sm_knl.h

-- 
Regards,

Laurent Pinchart