Re: [PATCH RFC] modpost: Support I2C Aliases from OF tables

[Javier Martinez Canillas <javierm@redhat.com>]

Hello Masahiro-san,

On 8/1/19 4:17 AM, Masahiro Yamada wrote:
> Hi.
> 
> On Thu, Aug 1, 2019 at 4:44 AM Wolfram Sang <wsa@the-dreams.de> wrote:
>>
>> Hi Javier,
>>
>> thank you for providing the extra information.
>>
>> (And Kieran, thanks for the patch!)
>>
>>> The other option is to remove i2c_of_match_device() and don't make OF match
>>> to fallback to i2c_of_match_device_sysfs(). This is what happens in the ACPI
>>> case, since i2c_device_match() just calls acpi_driver_match_device() directly
>>> and doesn't have a wrapper function that fallbacks to sysfs matching.
>>>
>>> In this case an I2C device ID table would be required if the devices have to
>>> be instantiated through sysfs. That way the I2C table would be used both for
>>> auto-loading and also to match the device when it doesn't have an of_node.
>>
>> That would probably mean that only a minority of drivers will not add an I2C
>> device ID table because it is easy to add an you get the sysfs feature?
>>
>> Then we are back again with the situation that most drivers will have
>> multiple tables. With the minor change that the I2C device id table is
>> not required anymore by the core, but it will be just very useful to
>> have? Or?
>>
>>> If the former is the correct way to solve this then the patch looks good to me.
>>>
>>> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
>>
>> For this actual patch from Kieran, I'd like to hear an opinion from the
>> people maintaining modpost.
> 
> 
> As you see 'git log scripts/mod/file2alias.c',
> this file is touched by every subsystem.
> 
> So, the decision is up to you, Wolfram.
> And, you can pick this to your tree if you like.
> 
> 
> The implementation is really trivial.
> 
> 
> As Javier pointed out, this discussion comes down to
> "do we want to fall back to i2c_of_match_device_sysfs()?"
> 

Yes, I think that's the crux of the matter. Basically the matching logic
should be consistent with the modalias uevent exposed to user-space to
auto-load modules.

So I think that we should either:

a) take Kieran's patch or b) remove the i2c_of_match_device_sysfs() fallback
for OF and require an I2C device table for sysfs instantiation and matching.

> If a driver supports DT and devices are instantiated via DT,
> in which situation is this useful?

Is useful if you don't have all the I2C devices described in the DT. For example
a daughterboard with an I2C device is connected to a board through an expansion
slot or an I2C device connected directly to I2C pins exposed in a machine.

In these cases your I2C devices won't be static so users might want to use the
sysfs user-space interface to instantiate the I2C devices, i.e:

 # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device

as explained in https://github.com/torvalds/linux/blob/master/Documentation/i2c/instantiating-devices#L207

> Do legacy non-DT platforms need this?

Yes, can also be used by non-DT platforms. But in this case isn't a problem
because drivers for these platform will already have an I2C device ID table.

> 
> 
> 
>> The aproach looks okay to me, yet I can't
>> tell how "easy" we are with adding new types like 'i2c_of'.
> 
> As far as I understood, this patch provides a shorthand.
> You can save one table, but still get the
> same MODULE_ALIAS in the *.mod.c file.
> You need to add two MODULE_DEVICE_TABLE() though.
> 
> MODULE_DEVICE_TABLE(of, si4713_of_match);
> MODULE_DEVICE_TABLE(i2c_of, si4713_of_match);
>

That's my understanding as well.

Best regards,
-- 
Javier Martinez Canillas
Software Engineer - Desktop Hardware Enablement
Red Hat