linux-kernel - Re: [PATCH v3 2/2] drm/bridge: tc358767: Expose test mode functionality via debugfs

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Date:   Mon, 9 Dec 2019 11:38:39 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Andrey Smirnov <andrew.smirnov@...il.com>,
        <dri-devel@...ts.freedesktop.org>
CC:     Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Cory Tusar <cory.tusar@....aero>,
        Chris Healy <cphealy@...il.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        <linux-kernel@...r.kernel.org>, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v3 2/2] drm/bridge: tc358767: Expose test mode
 functionality via debugfs

(Cc'ing Daniel for the last paragraph)

On 09/12/2019 07:08, Andrey Smirnov wrote:
> Presently, the driver code artificially limits test pattern mode to a
> single pattern with fixed color selection. It being a kernel module
> parameter makes switching "test pattern" <-> "proper output" modes
> on-the-fly clunky and outright impossible if the driver is built into
> the kernel.

That's not correct, /sys/module/tc358767/parameters/test is there even if the driver is built-in.

I think the bigger problems are that there's just one value, even if there are multiple devices, and 
that with kernel parameter the driver can't act on it dynamically (afaik).

> To improve the situation a bit, convert current test pattern code to
> use debugfs instead by exposing "TestCtl" register. This way old
> "tc_test_pattern=1" functionality can be emulated via:
> 
>      echo -n 0x78146302 > tstctl
> 
> and switch back to regular mode can be done with:
> 
>      echo -n 0x78146300 > tstctl

In the comment in the code you have 0 as return-to-regular-mode.

With my setup, enabling test mode seems to work, but when I return to regular mode, the first echo 
results in black display, but echoing 0 a second time will restore the display.

Hmm, actually, just echoing 0 to tstctl multiple times, it makes the screen go black and then 
restores it with every other echo.

> +	debugfs = debugfs_create_dir(dev_name(dev), NULL);
> +	if (!IS_ERR(debugfs)) {
> +		debugfs_create_file_unsafe("tstctl", 0200, debugfs, tc,
> +					   &tc_tstctl_fops);
> +		devm_add_action_or_reset(dev, tc_remove_debugfs, debugfs);
> +	}
> +

For me this creates debugfs/3-000f/tstctl. I don't think that's a clear or usable path, and could 
even cause a name conflict in the worst case.

Not sure what's a good solution here, but only two semi-good ones come to mind: have it in sysfs 
under the device's dir, or debugfs/dri/something. The latter probably needs some thought and common 
agreement on how to handle bridge and panel debugfs files. But that would be a good thing to have, 
as I'm sure there are other similar cases (at least I have a few).

  Tomi

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