linux-kernel - Re: [PATCH] imx-mipi-csis: Get the number of active lanes from mbus

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Message-ID: <aLbnjLapJXlCe67R@valkosipuli.retiisi.eu>
Date: Tue, 2 Sep 2025 15:48:12 +0300
From: Sakari Ailus <sakari.ailus@....fi>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Isaac Scott <isaac.scott@...asonboard.com>, linux-media@...r.kernel.org,
	rmfrfs@...il.com, martink@...teo.de, kernel@...i.sm,
	mchehab@...nel.org, shawnguo@...nel.org, s.hauer@...gutronix.de,
	kernel@...gutronix.de, festevam@...il.com, imx@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] imx-mipi-csis: Get the number of active lanes from
 mbus_config

Hi Laurent, Isaac,

On Tue, Sep 02, 2025 at 02:38:05PM +0200, Laurent Pinchart wrote:
> On Tue, Sep 02, 2025 at 01:28:37PM +0100, Isaac Scott wrote:
> > Quoting Laurent Pinchart (2025-08-19 03:44:13)
> > <snip>
> > > > > > > That would need to parse the endpoint every time we start streaming, it
> > > > > > > doesn't sound ideal.
> > > > > > 
> > > > > > Perhaps not, but does that matter in practice? Parsing the endpoint is,
> > > > > > after all, fairly trivial. The advantage would be simplifying drivers.
> > > > > 
> > > > > It's trivial from a code point of view, but it's not a cheap operation.
> > > > > I'd like to avoid making starting streaming more expensive.
> > > > 
> > > > How cheap is "not cheap"? I'd be surprised if parsing an endpoint took more
> > > > time than e.g. an I²C register write. Of course it depends on the CPU...
> > > 
> > > Still, it's not cheap, and I think it can easily be avoided.
> > > 
> > > > > > Alternatively we could think of caching this information somewhere but I
> > > > > > don't think it's worth it.
> > > > > 
> > > > > Drivers likely need to parse endpoints for other reasons. I'd cache the
> > > > > value in drivers, like done today, and pass it to a get_active_lanes
> > > > > helper.
> > > > 
> > > > Then drivers presumably would also validate this against the endpoint
> > > > configuration, wouldn't they? That's extra code in every CSI-2 receiver
> > > > driver.
> > > 
> > > Why so ? The number of connected lanes can be passed to the helper
> > > function, which can use it to validate the number of lanes reported by
> > > the source subdev.
> > 
> > Apologies if I'm interpreting this wrong, but it seems that the main
> > thing I'm reading is that this is not the correct place to implement
> > this, and it should be implemented at a higher level (e.g. in v4l2) that
> > lets all MIPI CSI reciever drivers use it?
> > 
> > I have noticed that similar functionality has been implemented as part
> > of __v4l2_get_link_freq_pad. Are you suggesting that I take a similar
> > approach and resubmit as a new series?
> 
> As far as iI understand, Sakari would like a helper function that will
> query the remote subdev for the number of data lanes it uses, and
> validates that against the number of connected data lanes as described
> by DT. I don't like the idea of parsing the endpoint properties every
> time we do so, so I think the number of connected data lanes should be
> passed by the driver to the helper instead. The helper would still query
> the remote subdev, and validate the value.

As long as the bulk of the work is done by the helper, I'm fine. This is a
fairly specific need but still in principle every CSI-2 receiver driver
needs it, so ease of use does count.

The helper could e.g. take the number of lanes in the endpoint as an
additional argument and just return the value if the sub-device doesn't
implement get_mbus_config() pad op. That'd be fairly trivial to use in a
driver.

> 
> > > > > > > > The function could take struct media_pad pointer as an argument, or struct
> > > > > > > > v4l2_subdev pointer and the pad number.
> > > > > > > > 
> > > > > > > > I wonder if any other parameters could change dynamically but I can't think
> > > > > > > > of that now, so perhaps just the number of lanes is what the function
> > > > > > > > should indeed return.
> > > > > > > > 
> > > > > > > > > +
> > > > > > > > > +   return 0;
> > > > > > > > > +}
> > > > > > > > > +
> > > > > > > > >  static int mipi_csis_s_stream(struct v4l2_subdev *sd, int enable)
> > > > > > > > >  {
> > > > > > > > >     struct mipi_csis_device *csis = sd_to_mipi_csis_device(sd);
> > > > > > > > > @@ -965,6 +1002,10 @@ static int mipi_csis_s_stream(struct v4l2_subdev *sd, int enable)
> > > > > > > > >     format = v4l2_subdev_state_get_format(state, CSIS_PAD_SINK);
> > > > > > > > >     csis_fmt = find_csis_format(format->code);
> > > > > > > > >  
> > > > > > > > > +   ret = mipi_csis_get_active_lanes(sd);
> > > > > > > > > +   if (ret < 0)
> > > > > > > > > +           dev_dbg(csis->dev, "Failed to get active lanes: %d", ret);
> > > > > > > > > +
> > > > > > > > >     ret = mipi_csis_calculate_params(csis, csis_fmt);
> > > > > > > > >     if (ret < 0)
> > > > > > > > >             goto err_unlock;

-- 
Kind regards,

Sakari Ailus