linux-kernel - Re: [PATCH 08/14] media: sunxi: Add support for the A31 MIPI CSI-2 controller

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Message-ID: <20201104184538.f6qagsmjdoijbzmv@gilmour.lan>
Date:   Wed, 4 Nov 2020 19:45:38 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Helen Koike <helen.koike@...labora.com>
Cc:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devel@...verdev.osuosl.org, linux-sunxi@...glegroups.com,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        Yong Deng <yong.deng@...ewell.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        Vinod Koul <vkoul@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Hans Verkuil <hverkuil@...all.nl>, kevin.lhopital@...mail.com
Subject: Re: [PATCH 08/14] media: sunxi: Add support for the A31 MIPI CSI-2
 controller

On Wed, Nov 04, 2020 at 01:38:08PM -0300, Helen Koike wrote:
> 
> 
> On 11/4/20 8:17 AM, Paul Kocialkowski wrote:
> > Hi,
> > 
> > On Mon 02 Nov 20, 10:21, Maxime Ripard wrote:
> >> On Fri, Oct 30, 2020 at 07:45:18PM -0300, Helen Koike wrote:
> >>> On 10/23/20 2:45 PM, Paul Kocialkowski wrote:
> >>>> The A31 MIPI CSI-2 controller is a dedicated MIPI CSI-2 controller
> >>>> found on Allwinner SoCs such as the A31 and V3/V3s.
> >>>>
> >>>> It is a standalone block, connected to the CSI controller on one side
> >>>> and to the MIPI D-PHY block on the other. It has a dedicated address
> >>>> space, interrupt line and clock.
> >>>>
> >>>> Currently, the MIPI CSI-2 controller is hard-tied to a specific CSI
> >>>> controller (CSI0) but newer SoCs (such as the V5) may allow switching
> >>>> MIPI CSI-2 controllers between CSI controllers.
> >>>>
> >>>> It is represented as a V4L2 subdev to the CSI controller and takes a
> >>>> MIPI CSI-2 sensor as its own subdev, all using the fwnode graph and
> >>>> media controller API.
> >>>
> >>> Maybe this is a bad idea, but I was thinking:
> >>> This driver basically just turn on/off and catch some interrupts for errors,
> >>> and all the rest of v4l2 config you just forward to the next subdevice
> >>> on the pipeline.
> >>>
> >>> So instead of exposing it as a subdevice, I was wondering if modeling
> >>> this driver also through the phy subsystem wouldn't be cleaner, so
> >>> you won't need all the v4l2 subdevice/topology boilerplate code that
> >>> it seems you are not using (unless you have plans to add controls or
> >>> some specific configuration on this node later).
> >>>
> >>> But this would require changes on the sun6i-csi driver.
> >>>
> >>> What do you think?
> >>
> >> Eventually we'll need to filter the virtual channels / datatypes I
> >> guess, so it's definitely valuable to have it in v4l2
> 
> Which kind of datatypes? 

MIPI-CSI datatypes. Each packet on the MIPI-CSI bus is assigned a
virtual channel and data type so that you can multiplex multiple streams
(like a 3d camera would send for example, through the virtual channels)
and data types (like frames and metadata) and MIPI-CSI controllers
usually allow to filter them based on what you want.

> I ask to know if this shouldn't be configured through the video node
> instead of subdevice.

Not really, some setups have a mux that can split the multiple virtual
channels to multiple video nodes for example.

> Regarding channels, we had a discussion to implement it through the video
> node (and not subdevice) [1]. But we discussed about blitters and multi-scalers,
> so now I'm wondering if we could use the same API for mipi-csi virtual channels
> in the video entity device, or if it doesn't apply and we need another API
> for that in a subdevice instead.
> 
> [1] https://patchwork.linuxtv.org/project/linux-media/cover/20200717115435.2632623-1-helen.koike@collabora.com/

There's already an API to deal with MIPI-CSI virtual channels:
https://patchwork.kernel.org/project/linux-renesas-soc/cover/20190328200608.9463-1-jacopo+renesas@jmondi.org/

Maxime

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