linux-kernel - Re: [PATCH 13/18] ipu3-cio2: Add functionality allowing software_node connections to sensors on platforms designed for Windows

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Date:   Tue, 15 Dec 2020 10:28:59 +0000
From:   Daniel Scally <djrscally@...il.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-gpio@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-media@...r.kernel.org, devel@...ica.org, rjw@...ysocki.net,
        lenb@...nel.org, gregkh@...uxfoundation.org,
        mika.westerberg@...ux.intel.com, andriy.shevchenko@...ux.intel.com,
        linus.walleij@...aro.org, bgolaszewski@...libre.com,
        wsa@...nel.org, yong.zhi@...el.com, sakari.ailus@...ux.intel.com,
        bingbu.cao@...el.com, tian.shu.qiu@...el.com, mchehab@...nel.org,
        robert.moore@...el.com, erik.kaneda@...el.com, pmladek@...e.com,
        rostedt@...dmis.org, sergey.senozhatsky@...il.com,
        linux@...musvillemoes.dk, kieran.bingham+renesas@...asonboard.com,
        jacopo+renesas@...ndi.org,
        laurent.pinchart+renesas@...asonboard.com,
        jorhand@...ux.microsoft.com, kitakar@...il.com,
        heikki.krogerus@...ux.intel.com
Subject: Re: [PATCH 13/18] ipu3-cio2: Add functionality allowing software_node
 connections to sensors on platforms designed for Windows

Morning Sakari

On 30/11/2020 20:35, Sakari Ailus wrote:
>> +/*
>> + * Extend this array with ACPI Hardware ID's of devices known to be working.
>> + * Do not add a HID for a sensor that is not actually supported.
>> + */
>> +static const char * const cio2_supported_devices[] = {
>> +	"INT33BE",
>> +	"OVTI2680",
> 
> I guess we don't have the known-good frequencies for the CSI-2 bus in
> firmware?
> 
> One option would be to put there what the drivers currently use. This
> assumes the support for these devices is, well, somewhat opportunistic but
> I guess there's no way around that right now at least.
> 
> As the systems are laptops, they're likely somewhat less prone to EMI
> issues to begin with than mobile phones anyway.

Just looking at this; we're currently using this with the ov2680 driver
that's in mainline currently (with very minor tweaks) plus a
hacked-into-roughly-working version of the atomisp-ov5693 driver (ACPI
ID INT33BE = ov5693 physical device). Neither of those drivers lists any
link frequencies, nor provides a link frequency control for v4l2 to work
with.

On the other hand, the ov5648 [1] and ov8865 [2] drivers which Paul has
submitted recently, which we also want to be able to support, _do_
include that. I can register the frequencies Paul's defined there as a
link-frequencies property but this gives rise to two questions:

1. Is this _mandatory_? Do I need to be finding the link-frequencies for
the OV2680 and OV5693 drivers too? Or can I skip that property where the
driver doesn't handle it anyway. Seems to be working fine without
controlling it in driver.
2. Can I trust all the values in the drivers to be available on each
platform? For example for the ov5648 Paul lists these as available:

 938static const s64 ov5648_link_freq_menu[] = {

 939        210000000,

 940        168000000,

 941};

But can I safely register a link-frequencies property for both of those
and trust that that'll work on all IPU3 platforms with an ov5648 in them?

Thanks
Dan