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Date:   Mon, 19 Sep 2022 16:49:06 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Sakari Ailus <sakari.ailus@linux.intel.com>
Cc:     Mikhail Rudenko <mike.rudenko@gmail.com>,
        Dave Stevenson <dave.stevenson@raspberrypi.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Jacopo Mondi <jacopo@jmondi.org>,
        Shawn Tu <shawnx.tu@intel.com>, Jimmy Su <jimmy.su@intel.com>,
        Arnd Bergmann <arnd@arndb.de>, Arec Kao <arec.kao@intel.com>,
        Marek Vasut <marex@denx.de>, linux-media@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/2] Add Omnivision OV4689 image sensor driver
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Hello,

On Mon, Sep 19, 2022 at 10:31:02AM +0000, Sakari Ailus wrote:
> On Mon, Sep 19, 2022 at 10:01:06AM +0300, Mikhail Rudenko wrote:
> > On 2022-09-19 at 06:40 GMT, Sakari Ailus wrote:
> > > On Fri, Sep 16, 2022 at 12:27:42AM +0300, Mikhail Rudenko wrote:
> > >> On 2022-09-14 at 10:58 +01, Dave Stevenson wrote:
> > >> > On Sun, 11 Sept 2022 at 21:02, Mikhail Rudenko wrote:
> > >> >>
> > >> >> Hello,
> > >> >>
> > >> >> this series implements support for Omnivision OV4689 image
> > >> >> sensor. The Omnivision OV4689 is a high performance, 1/3-inch, 4
> > >> >> megapixel image sensor. Ihis chip supports high frame rate speeds up
> > >> >> to 90 fps at 2688x1520 resolution. It is programmable through an I2C
> > >> >> interface, and sensor output is sent via 1/2/4 lane MIPI CSI-2
> > >> >> connection.
> > >> >>
> > >> >> The driver is based on Rockchip BSP kernel [1]. It implements 4-lane CSI-2
> > >> >> and single 2688x1520 @ 30 fps mode. The driver was tested on Rockchip
> > >> >> 3399-based FriendlyElec NanoPi M4 board with MCAM400 camera
> > >> >> module.
> > >> >> While porting the driver, I stumbled upon two issues:
> > 
> > [snip]
> > 
> > >> >> (2) The original driver exposes analog gain range 0x0 - 0x7ff, but the
> > >> >> gain is not linear across that range. Instead, it is piecewise linear
> > >> >> (and discontinuous). 0x0-0xff register values result in 0x-2x gain,
> > >> >> 0x100-0x1ff to 0x-4x, 0x300-0x3ff to 0x-8x, and 0x700-0x7ff to 0x-16x,
> > >> >> with more linear segments in between. Rockchip's camera engine code
> > >> >> chooses one of the above segments depenging on the desired gain
> > >> >> value. The question is, how should we proceed keeping in mind
> > >> >> libcamera use case? Should the whole 0x0-0x7ff be exposed as-is and
> > >> >> libcamera will do the mapping, or the driver will do the mapping
> > >> >> itself and expose some logical gain units not tied to the actual gain
> > >> >> register value? Meanwhile, this driver conservatively exposes only
> > >> >> 0x0-0xf8 gain register range.
> > >> >
> > >> > The datasheet linked above says "for the gain formula, please contact
> > >> > your local OmniVision FAE" :-(
> > >> > I would assume that the range is from 1x rather than 0x - people
> > >> > rarely want a totally black image that 0x would give. Or is it ranges
> > >> > of 1x - 2x, 2x - 4x, 4x - 8x, and 8x - 16x?
> > >>
> > >> A picture is worth a thousand words, so I've attached the results of my
> > >> experimentation with the gain register. They were obtained with Rockchip
> > >> 3399, with AEC, AGC and black level subtraction disabled. The image was
> > >> converted from 10-bit RGGB to 8-bit YUV 4:2:0 by the Rockchip ISP.

Is that full or limited range YUV ?

> > > Based on that it looks like their medication may have been a little too
> > > strong.
> > >
> > > Could this be implemented so that the control value would be linear linear
> > > but its range would correspond 1x--16x values?
> > >
> > > libcamera will be able to cope with that.
> > 
> > According to the following fragment of the Rockchip camera engine sensor
> > configuration file for ov4689 [1]
> > 
> >     <Linear index="1" type="double" size="[4 7]">
> >        [1 2 128 0 1 128 255
> >         2 4 64 -248 1 376 504
> >         4 8 32 -756 1 884 1012
> >         8 16 16 -1784 1 1912 2040]
> >     </Linear>,
> > 
> > it uses gain register value range 128-255 for gain 1x-2x, 376-504 for
> > gain 2x-4x, 884-1024 for 4x-8x, and 1912-2040 for 8x-16x. Do you suggest

That looks *really* weird. I would have understood [384, 511], [896,
1023] and [1920, 2047], but not those intervals.

The driver hardcodes bit 0x3503[2] to 1, which means "sensor gain
format". Maybe setting it to 0 ("real gain format") would produce saner
results ?

> > to implement this calculation in the sensor driver and expose some
> > linear "logical" gain to userspace (ranging, e.g., 128-2048 for gains
> > 1x-16x)?
> 
> Yes. This way the user space can somehow work without knowing this special
> implementation, even though the granularity changes over the range. I guess
> the granularity would need to be known in libcamera but that's a separate
> issue.

I can live with that.

-- 
Regards,

Laurent Pinchart