linux-kernel - Re: [PATCH v2 05/15] staging: mmal-vchiq: Use vc-sm-cma to support zero copy

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Message-ID: <20231115192705.GC21100@pendragon.ideasonboard.com>
Date:   Wed, 15 Nov 2023 21:27:05 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Umang Jain <umang.jain@...asonboard.com>,
        linux-media@...r.kernel.org, kernel-list@...pberrypi.com,
        linux-kernel@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-staging@...ts.linux.dev,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        "Ricardo B . Marliere" <ricardo@...liere.net>,
        Dan Carpenter <error27@...il.com>
Subject: Re: [PATCH v2 05/15] staging: mmal-vchiq: Use vc-sm-cma to support
 zero copy

On Wed, Nov 15, 2023 at 08:03:47PM +0100, Stefan Wahren wrote:
> Hi Umang,
> 
> Am 09.11.23 um 22:02 schrieb Umang Jain:
> > From: Dave Stevenson <dave.stevenson@...pberrypi.com>
> > 
> > With the vc-sm-cma driver we can support zero copy of buffers between
> > the kernel and VPU. Add this support to mmal-vchiq.
> > 
> > Signed-off-by: Dave Stevenson <dave.stevenson@...pberrypi.com>
> > Signed-off-by: Umang Jain <umang.jain@...asonboard.com>
> > ---
> >   .../staging/vc04_services/vchiq-mmal/Kconfig  |  1 +
> >   .../vc04_services/vchiq-mmal/mmal-common.h    |  4 +
> >   .../vc04_services/vchiq-mmal/mmal-vchiq.c     | 84 +++++++++++++++++--
> >   .../vc04_services/vchiq-mmal/mmal-vchiq.h     |  1 +
> >   4 files changed, 83 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/staging/vc04_services/vchiq-mmal/Kconfig b/drivers/staging/vc04_services/vchiq-mmal/Kconfig
> > index c99525a0bb45..a7c1a7bf516e 100644
> > --- a/drivers/staging/vc04_services/vchiq-mmal/Kconfig
> > +++ b/drivers/staging/vc04_services/vchiq-mmal/Kconfig
> > @@ -1,6 +1,7 @@
> >   config BCM2835_VCHIQ_MMAL
> >   	tristate "BCM2835 MMAL VCHIQ service"
> >   	depends on BCM2835_VCHIQ
> > +	select BCM_VC_SM_CMA
> 
> i think we need more explanation in the commit message of the relation 
> between these both modules.
> 
> On the one side BCM_VC_SM_CMA should be a driver, but it's not a driver 
> for a specific hardware. It looks like more an extension of VCHIQ MMAL 
> or does other (maybe not yet imported) vc04 driver make also use of this.
> 
> My question is: is BCM_VC_SM_CMA a real member on the VCHIQ bus and why?

The vchiq_device pointer passed to the vc_sm_cma probe function is used
in the driver for the sole purpose of accessing the underlying struct
device, which, beside being used for dev_*() log calls, is just passed
to dma_buf_attach(). It does indeed appear to be more of a service than
a device.

-- 
Regards,

Laurent Pinchart