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 17:05:19 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Andrey Smirnov <andrew.smirnov@...il.com>
CC:     <dri-devel@...ts.freedesktop.org>,
        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 <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

On 09/12/2019 16:38, Andrey Smirnov wrote:
> On Mon, Dec 9, 2019 at 1:38 AM Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>>
>> (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.
>>
> 
> True, I'll drop the "impossible" part of the descrption. Having to
> unbind and bind device to the driver to use that parameter definitely
> falls under "clunky" for me still, so I'll just stick to that in the
> description.

You don't need to re-bind. You can change the module parameter at runtime, and if the driver happens 
to use the value, then it uses the new value. If I recall right, changing the module parameter and 
then doing a full modeset from userspace made the driver to use the test mode (I'm not 100% sure, 
though).

In any case, I'm not advocating for the use of module parameter here =)

>> Hmm, actually, just echoing 0 to tstctl multiple times, it makes the screen go black and then
>> restores it with every other echo.
>>
> 
> Strange, works on my setup every time. No error messages in kernel log
> I assume? Probably unrelated, but when you echo "0" and the screen

No errors.

> stays black, what do you see in DP_SINK_STATUS register:
> 
> dd if=/dev/drm_dp_aux0 bs=1 skip=$((0x205)) count=1 2>/dev/null | hexdump -Cv
> 
> ? Note that this needs CONFIG_DRM_DP_AUX_CHARDEV to be enabled.

I'll check this later, and do a few more tests.

>>> +     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.
>>
> 
> I agree on usability aspect, but I am not sure I can see how a
> conflict can happen. What scenario do you have in mind that would
> cause that? My thinking was that the combination of I2C bus number +
> I2C address should always be unique on the system, but maybe I am
> missing something?

Well, the dir name doesn't have "i2c" anywhere, so at least in theory, some other bus could have 
"3-000f" address too.

Maybe bigger problem is that it's not at all obvious what "3-000f" means. All the other debugfs dirs 
make sense when you look at the name, and "3-000f" looks very odd there.

  Tomi

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