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

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Date:   Mon, 20 Mar 2023 16:00:12 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        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 Mon, Mar 20, 2023 at 02:32:25PM +0100, Philipp Zabel wrote:
> On Mo, 2023-03-20 at 12:15 +0200, Sakari Ailus wrote:
> > On Mon, Mar 20, 2023 at 11:55:14AM +0200, Laurent Pinchart wrote:
> > > On Mon, Mar 20, 2023 at 11:37:33AM +0200, Sakari Ailus wrote:
> > > > 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 g_mbus_config op could be implemented by the CSI2IPU and mux, by simply
> > requesting the information from the upstream sub-device. No hacks would be
> > needed.
> 
> I think implementing get_mbus_config on the mux might be enough. The
> IPU driver already recognizes the CSI-2 RX by its grp_id and could
> infer that it has to use MIPI CSI-2 mode from that.
> 
> The video-mux would have to forward get_mbus_config to its active
> upstream port and if the upstream sensor does not implement it read bus
> width from the active upstream endpoint.

I'm fine with implementing it in the mux as well, but I think we can
take a shortcut here and call it on the CSI-2 RX from the IPU driver, as
the IPU driver knows about the architecture of the whole pipeline.

-- 
Regards,

Laurent Pinchart