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

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Date:   Wed, 15 Nov 2023 20:03:47 +0100
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     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
Cc:     Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        Laurent Pinchart <laurent.pinchart@...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

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?

Best regards