Date:   Fri, 20 May 2022 16:57:35 +0200
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-staging@...ts.linux.dev,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v3 3/4] staging: media: Add support for the Allwinner A31
 ISP

Hi Sakari,

On Fri 29 Apr 22, 00:07, Sakari Ailus wrote:
> Hi Paul,
> 
> On Thu, Apr 28, 2022 at 04:30:11PM +0200, Paul Kocialkowski wrote:
> > Hi Sakari,
> > 
> > On Thu 28 Apr 22, 15:14, Sakari Ailus wrote:
> > > Hi Paul,
> > > 
> > > Thanks for the set.
> > > 
> > > A few comments below.
> > 
> > Thanks a lot for your review!
> 
> You're welcome!
> 
> ...
> 
> > > I understand this is an online ISP. How do you schedule the video buffer
> > > queues? Say, what happens if it's time to set up buffers for a frame and
> > > there's a buffer queued in the parameter queue but not in the image data
> > > queue? Or the other way around?
> > 
> > The ISP works in a quite atypical way, with a DMA buffer that is used to
> > hold upcoming parameters (including buffer addresses) and a bit in a "direct"
> > register to schedule the update of the parameters at next vsync.
> > 
> > The update (setting the bit) is triggered whenever new parameters are
> > submitted via the params video device or whenever there's a capture buffer
> > available in the capture video device.
> > 
> > So you don't particularly need to have one parameter buffer matching a capture
> > buffer, the two can be updated independently. Of course, a capture buffer will
> > only be returned after another buffer becomes active.
> 
> This also means it's not possible to associate a capture buffer to a
> parameter buffer by other means than timing --- which is unreliable. The
> request API would allow that but it's not free of issues either.

Yes the request API seems like a good fit for this. Note that the returned
sequence number in dequeued buffers for the capture and meta video devices
should match though, so userspace still has a way to know which captured buffer
used parameters from which meta params buffer.

> Alternatively, I think in this case you could always require the capture
> buffer and grab a parameter buffer when it's available. As ISPs are
> generally requiring device specific control software, this shouldn't be a
> problem really.

I think this is pretty much what happens already.

> I wonder what Laurent thinks.
> 
> > 
> > I hope this answers your concern!
> > 
> > [...]
> > 
> > > > +static int sun6i_isp_tables_setup(struct sun6i_isp_device *isp_dev)
> > > > +{
> > > > +	struct sun6i_isp_tables *tables = &isp_dev->tables;
> > > > +	int ret;
> > > > +
> > > > +	/* Sizes are hardcoded for now but actually depend on the platform. */
> > > 
> > > Would it be cleaner to have them defined in a platform-specific way, e.g.
> > > in a struct you obtain using device_get_match_data()?
> > 
> > Absolutely! I didn't do it at this stage since only one platform is supported
> > but we could just as well introduce a variant structure already for the table
> > sizes.
> 
> I think that would be nice already, especially if you know these are going
> to be different. Otherwise macros could be an option.

Understood!

> ...
> 
> > > > +	ret = v4l2_ctrl_handler_init(&v4l2->ctrl_handler, 0);
> > > 
> > > I suppose you intend to add controls later on?
> > 
> > I might be wrong but I thought this was necessary to expose sensor controls
> > registered by subdevs that end up attached to this v4l2 device.
> > 
> > I doubt the drivers itself will expose controls otherwise.
> 
> Now that this is an MC-enabled driver, the subdev controls should be
> accessed through the subdev nodes only. Adding them to the video device's
> control handler is quite hackish and not guaranteed to even work (as e.g.
> multiple subdevs can have the same control).

Yes I was wondering what would happen in that case. I'll drop the ctrls
handling in the next iteration then.

Paul

> ...
> 
> > > > +{
> > > > +	struct sun6i_isp_device *isp_dev = video_drvdata(file);
> > > > +	struct video_device *video_dev = &isp_dev->capture.video_dev;
> > > > +	struct mutex *lock = &isp_dev->capture.lock;
> > > > +	int ret;
> > > > +
> > > > +	if (mutex_lock_interruptible(lock))
> > > > +		return -ERESTARTSYS;
> > > > +
> > > > +	ret = v4l2_pipeline_pm_get(&video_dev->entity);
> > > 
> > > Do you need this?
> > > 
> > > Drivers should primarily depend on runtime PM, this is only needed for
> > > compatibility reasons. Instead I'd like to see sensor drivers being moved
> > > to runtime PM.
> > 
> > Yes it's still needed to support sensor drivers that don't use rpm yet.
> 
> To that I suggested adding runtime PM support for the affected sensors.
> This doesn't seem to get done otherwise. E.g. ipu3-cio2 driver does not
> call s_power() on sensor subdevs.
> 
> ...
> 
> > > > +	ret = video_register_device(video_dev, VFL_TYPE_VIDEO, -1);
> > > > +	if (ret) {
> > > > +		v4l2_err(v4l2_dev, "failed to register video device: %d\n",
> > > > +			 ret);
> > > > +		goto error_media_entity;
> > > > +	}
> > > > +
> > > > +	v4l2_info(v4l2_dev, "device %s registered as %s\n", video_dev->name,
> > > > +		  video_device_node_name(video_dev));
> > > 
> > > This isn't really driver specific. I'd drop it.
> > 
> > I agree but I see that many drivers are doing it and the information can
> > actually be quite useful at times.
> 
> You can get that information using media-ctl -e 'entity name'.
> 
> I guess this could be also added to video_register_device() on debug level.
> 
> > > > +struct sun6i_isp_params_config_bdnf {
> > > > +	__u8	in_dis_min; // 8
> > > > +	__u8	in_dis_max; // 10
> > > 
> > > Are these default values or something else? Better documentation was in the
> > > TODO.txt file already.
> > 
> > Yes that's the default register values, but these comments are and overlook on
> > my side and should be removed.
> 
> I'm fine leaving these here. Just wondering. Up to you.
> 
> -- 
> Kind regards,
> 
> Sakari Ailus

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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