linux-kernel - Re: [PATCH v4 23/36] media: imx: Add MIPI CSI-2 Receiver subdev driver

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Message-ID: <5ebd3f35-a7b7-bbbe-4d40-59de3575ded2@gmail.com>
Date:   Wed, 22 Feb 2017 15:41:18 -0800
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Philipp Zabel <p.zabel@...gutronix.de>,
        Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     robh+dt@...nel.org, mark.rutland@....com, shawnguo@...nel.org,
        kernel@...gutronix.de, fabio.estevam@....com, mchehab@...nel.org,
        hverkuil@...all.nl, nick@...anahar.org, markus.heiser@...marIT.de,
        laurent.pinchart+renesas@...asonboard.com, bparrot@...com,
        geert@...ux-m68k.org, arnd@...db.de, sudipm.mukherjee@...il.com,
        minghsiu.tsai@...iatek.com, tiffany.lin@...iatek.com,
        jean-christophe.trotin@...com, horms+renesas@...ge.net.au,
        niklas.soderlund+renesas@...natech.se, robert.jarzmik@...e.fr,
        songjun.wu@...rochip.com, andrew-ct.chen@...iatek.com,
        gregkh@...uxfoundation.org, shuah@...nel.org,
        sakari.ailus@...ux.intel.com, pavel@....cz,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>
Subject: Re: [PATCH v4 23/36] media: imx: Add MIPI CSI-2 Receiver subdev
 driver



On 02/22/2017 03:38 PM, Steve Longerbeam wrote:
>
>
> On 02/17/2017 03:38 AM, Philipp Zabel wrote:
>> On Fri, 2017-02-17 at 11:06 +0000, Russell King - ARM Linux wrote:
>>> On Fri, Feb 17, 2017 at 11:47:59AM +0100, Philipp Zabel wrote:
>>>> On Wed, 2017-02-15 at 18:19 -0800, Steve Longerbeam wrote:
>>>>> +static void csi2_dphy_init(struct csi2_dev *csi2)
>>>>> +{
>>>>> +    /*
>>>>> +     * FIXME: 0x14 is derived from a fixed D-PHY reference
>>>>> +     * clock from the HSI_TX PLL, and a fixed target lane max
>>>>> +     * bandwidth of 300 Mbps. This value should be derived
>>>>
>>>> If the table in https://community.nxp.com/docs/DOC-94312 is correct,
>>>> this should be 850 Mbps. Where does this 300 Mbps value come from?
>>>
>>> I thought you had some code to compute the correct value, although
>>> I guess we've lost the ability to know how fast the sensor is going
>>> to drive the link.
>>
>> I had code to calculate the number of needed lanes from the bit rate and
>> link frequency. I did not actually change the D-PHY register value.
>> And as you pointed out, calculating the number of lanes is not useful
>> without input from the sensor driver, as some lane configurations might
>> not be supported.
>>
>>> Note that the IMX219 currently drives the data lanes at 912Mbps almost
>>> exclusively, as I've yet to finish working out how to derive the PLL
>>> parameters.  (I have something that works, but it currently takes on
>>> the order of 100k iterations to derive the parameters.  gcd() doesn't
>>> help you in this instance.)
>>
>> The tc358743 also currently only implements a fixed rate (of 594 Mbps).
>
> I've analyzed the OV5640 video modes, and they generate the following
> "sysclk"'s in Mhz:
>
> 210
> 280
> 420
> 560
> 840
>
> But I don't know whether this is equivalent to bit rate. Is it the same
> as Mbps-per-lane? If so, this could be indicated to the sink by
> implementing V4L2_CID_LINK_FREQ in the ov5640.c sensor driver.
>

er, rather, if I knew what this "sysclk" value was, I could use it
to convert to a link frequency and return it in V4L2_CID_LINK_FREQ.

Steve

> The Mbps-per-lane value would then be link_freq * 2, and then
> Mbps-per-lane could be used to lookup the correct "hsfreqsel"
> value to program the D-PHY.
>
> I've added this to imx6-mipi-csi2.c. If the source didn't implement
> V4L2_CID_LINK_FREQ then it uses a default 849 Mbps-per-lane.
>
>
> Steve
>