linux-kernel - Re: [PATCH v2] media: i2c: ov5640: Implement get_mbus

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Message-ID: <9b4f0896a3a3e97b44197de263c30f0d31333abd.camel@pengutronix.de>
Date:   Mon, 20 Mar 2023 12:26:26 +0100
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Jacopo Mondi <jacopo.mondi@...asonboard.com>,
        Francesco Dolcini <francesco@...cini.it>,
        Sakari Ailus <sakari.ailus@....fi>,
        Marcel Ziswiler <marcel@...wiler.com>,
        linux-media@...r.kernel.org, kernel@...gutronix.de,
        Francesco Dolcini <francesco.dolcini@...adex.com>,
        Aishwarya Kothari <aishwarya.kothari@...adex.com>,
        Marcel Ziswiler <marcel.ziswiler@...adex.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Steve Longerbeam <slongerbeam@...il.com>,
        linux-kernel@...r.kernel.org,
        Marco Felsch <m.felsch@...gutronix.de>
Subject: Re: [PATCH v2] media: i2c: ov5640: Implement get_mbus_config

On Mo, 2023-03-20 at 11:55 +0200, Laurent Pinchart wrote:
> On Mon, Mar 20, 2023 at 11:37:33AM +0200, Sakari Ailus wrote:
> > Hi Laurent,
> > 
> > On Mon, Mar 20, 2023 at 11:26:02AM +0200, Laurent Pinchart wrote:
> > > In a (simplified) nutshell,
> > > 
> > > ---------+     +----------+     +---------+     +-----+     +-----+
> > > > Camera | --> | CSI-2 RX | --> | CSI2IPU | --> | Mux | --> | IPU |
> > > > Sensor |     |          |     | Gasket  |     |     |     |     |
> > > ---------+     +----------+     +---------+     +-----+     +-----+
> > 
> > Thank you, this is helpful.
> > 
> > I suppose the mux here at least won't actively do anything to the data. So
> > presumably its endpoint won't contain the active configuration, but its
> > superset.
> > 
> > > 
> > > All those blocks, except for the gasket, have a node in DT.
> > > 
> > > The IPU driver needs to know the number of CSI-2 data lanes, which is
> > > encoded in the data-lanes DT property present in both the sensor output
> > > endpoint and the CSI-2 RX input endpoint, but not the other endpoints in
> > > the pipeline.
> > 
> > This doesn't yet explain why the sensor would need to implement
> > get_mbus_config if its bus configuration remains constant.
> 
> If I recall correctly, the IPU driver calls .g_mbus_config() on the
> camera sensor to get the number of lanes, as it can't get it from its
> own endpoint. That's a hack, and as Jacopo proposed, calling
> .g_mbus_config() on the CSI-2 RX would be better, as the CSI-2 RX driver
> can then get the value from its own endpoint, without requiring all
> sensor drivers to implement .g_mbus_config().

The IPU driver doesn't call get_mbus_config, the CSI-2 RX driver does
(csi2_get_active_lanes in imx6-mipi-csi2.c). It could just fall back to
looking at its own endpoint if the upstream driver does not implement
get_mbus_config.

Of course that will only help if the DT contains this information and
all connected lanes are active.

regards
Philipp