linux-kernel - Re: [PATCH v1 2/3] auxdisplay: ht16k33: Make use of device_get_match

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Date:   Wed, 22 Feb 2023 17:46:00 +0100
From:   Robin van der Gracht <robin@...tonic.nl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Raul E Rangel <rrangel@...omium.org>,
        Wolfram Sang <wsa@...nel.org>, linux-kernel@...r.kernel.org,
        linux-i2c@...r.kernel.org, linux-usb@...r.kernel.org,
        Miguel Ojeda <ojeda@...nel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v1 2/3] auxdisplay: ht16k33: Make use of
 device_get_match_data()

On 2023-02-21 18:48, Andy Shevchenko wrote:
> On Tue, Feb 21, 2023 at 05:10:00PM +0100, Robin van der Gracht wrote:
>> On 2023-02-21 14:40, Andy Shevchenko wrote:
>> > On Tue, Feb 21, 2023 at 03:33:06PM +0200, Andy Shevchenko wrote:
>> > > Switching to use device_get_match_data() helps getting of
>> > > i2c_of_match_device() API.
> 
> ...
> 
>> > > -	id = i2c_of_match_device(dev->driver->of_match_table, client);
>> > > -	if (id)
>> > > -		priv->type = (uintptr_t)id->data;
>> > > +	priv->type = (uintptr_t)device_get_match_data(dev);
>> >
>> > Looking closer the I²C ID table should provide DISP_MATRIX to keep
>> > default and this needs to be not dropped.
>> >
>> > So, the question is what to do with unknown type then, return -EINVAL
>> > from probe()?
>> 
>> If you leave out your addition of the DISP_UNKNOWN type, the default 
>> type
>> will be DISP_MATRIX if no match is found, which is as it is now.
>> 
>> In that case the following change should suffice:
>> 
>> @@ -713,7 +715,6 @@ static int ht16k33_seg_probe(struct device *dev, 
>> struct
>> ht16k33_priv *priv,
>>  static int ht16k33_probe(struct i2c_client *client)
>>  {
>>      struct device *dev = &client->dev;
>> -    const struct of_device_id *id;
>>      struct ht16k33_priv *priv;
>>      uint32_t dft_brightness;
>>      int err;
>> @@ -728,9 +729,8 @@ static int ht16k33_probe(struct i2c_client 
>> *client)
>>          return -ENOMEM;
>> 
>>      priv->client = client;
>> -    id = i2c_of_match_device(dev->driver->of_match_table, client);
>> -    if (id)
>> -        priv->type = (uintptr_t)id->data;
>> +    priv->type = (uintptr_t)device_get_match_data(dev);
>> +
>>      i2c_set_clientdata(client, priv);
>> 
>>      err = ht16k33_initialize(priv);
>> 
>> Or do you think falling back to DISP_MATRIX if no match is found is 
>> wrong?
> 
> First of all, the I²C ID table should actually use DISP_MATRIX.
> 
> Second, there are two points:
> 
> - It would be nice to check if the OF ID table doesn't provide a 
> setting
>   (shouldn't we try I²C ID table and then, if still nothing, bail out?)
> 
> - The I²C ID table can be extended in the future with another entry 
> which
>   may want to have different default

For my understanding, please correct me if I'm wrong;

For all methods of instantiation during ht16k33 probe, 
i2c_of_match_device()
matches the compatible strings in the OF ID table due to a call to
i2c_of_match_device_sysfs().

device_get_match_data() only matches the compatible strings in the OF ID
table for devicetree instantiation because of_match_device() won't match
is there is no actual of_node.

So with only device_get_match_data() and a non devicetree instantiation,
priv->type will always be (uintptr_t)NULL = 0 = DISP_MATRIX.

Which effectively breaks i.e. user-space instantiation for other display
types which now do work due to i2c_of_match_device().
(so my suggestion above is not sufficient).

Are you proposing extending and searching the I2C ID table to work 
around that?

Kind regards,

-- 
Robin van der Gracht
Protonic Holland
Factorij 36
1689AL Zwaag
+31 (0)229 212928
https://www.protonic.nl