Message-ID: <f42f69dc-0c0b-64bc-fe4c-7ce7a9bb9a4e@redhat.com>
Date:   Sat, 16 Dec 2017 20:59:06 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>,
        linux-integrity@...r.kernel.org, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH] tpm: remove type and name fields from the I2C Infineon OF
 table entries

On 12/15/2017 01:03 PM, Jarkko Sakkinen wrote:
> Hi
> 
> Some basic questions about DT, which I'm not expert of.
> 
> On Mon, Dec 04, 2017 at 06:39:16PM +0100, Javier Martinez Canillas wrote:
>> The commit 21dc02eab989 ("tpm/tpm_i2c_infineon.c: Add OF attributes type
>> and name to the of_device_id table entries") added type and name fields
>> to the OF device ID table entries for the I2C Infineon TPM driver.
>>
>> The only justification for the change in the commit message is that it's
>> probably a good idea to have these fields populated in the OF entries.
>>
>> But I don't think that's true. In fact, I believe that's not correct to
>> add these fields for the following reasons:
>>
>>  * The struct of_device_id .type field matches the device_type property
>>    in the Device Tree nodes. The ePAPR document says that new use of the
>>    property is deprecated and should only be included in cpu and memory
>>    nodes for compatibility with the IEEE 1275-derived Device Trees.
>>    Also, mainline Device Tree source files don't include this property
>>    in the TPM nodes defined.
>>
>>  * The struct of_device_id .name field matches the Device Tree node name
>>    but the ePAPR document says that the node name should be generic and
>>    reflect the function of the device, not it's programming model. So in
>>    the case of TPM chips, it should just be "tpm" but the name fields are
>>    set in the OF table entries to the actual device model (i.e: slb9645tt).
> 
> So why the name field is not changed to "tpm"?
>

We could, but why would do that? Drivers usually don't care about the node name
used by the DTS and most DT bindings are not strict about that. There are some
exceptions (e.g: regulators bindings) but in most cases is up to the developer
to define whatever node name is suitable for that DT node.

In fact, this driver is the first one I have ever seen that sets a .name field.
What most drivers only use to match is the compatible string.

Also, it seems that "tpm" is not the only used node name for TPM device nodes.
At least "tpm_tis" and "vtpm" are used by looking at mainline DTS and bindings
documents. And who knows if an out-of-tree DTB uses "tpm2" for example?

Finally, the other two drivers for I2C TPM devices (atmel and nuvoton) don't
set a .name, so if we change in this driver we probably would need to change
in those too for consistency. And have a proper DT binding doc so users know
that using "tpm" as node name is a requirement.

>> Now, from a practical point of view this means that the OF module aliases
>> for the supported devices include the name and type of the device entries:
>>
>> $ modinfo drivers/char/tpm/tpm_i2c_infineon.ko | grep alias
>> alias:          of:Nslb9645ttTtpm*Cinfineon,slb9645ttC*
>> alias:          of:Nslb9645ttTtpm*Cinfineon,slb9645tt
>> alias:          of:Nslb9635ttTtpm*Cinfineon,slb9635ttC*
>> alias:          of:Nslb9635ttTtpm*Cinfineon,slb9635tt
>> alias:          of:Ntpm_i2c_infineonTtpm*Cinfineon,tpm_i2c_infineonC*
>> alias:          of:Ntpm_i2c_infineonTtpm*Cinfineon,tpm_i2c_infineon
>>
>> But since the device_type isn't set in the tpm DT nodes and the node name
>> is a generic one, the reported MODALIAS when a device is registered via OF
>> won't match the driver's module aliases:
>>
>> $ cat /sys/class/tpm/tpm0/device/modalias
>> of:NtpmT<NULL>Cinfineon,slb9645tt
>>
>> So remove these fields from the OF entries to allow the module aliases to
>> match the MODALIAS reported by the kernel when the device is registered:
>>
>> $ modinfo drivers/char/tpm/tpm_i2c_infineon.ko | grep alias
>> alias:          of:N*T*Cinfineon,slb9645ttC*
>> alias:          of:N*T*Cinfineon,slb9645tt
>> alias:          of:N*T*Cinfineon,slb9635ttC*
>> alias:          of:N*T*Cinfineon,slb9635tt
>> alias:          of:N*T*Cinfineon,tpm_i2c_infineonC*
>> alias:          of:N*T*Cinfineon,tpm_i2c_infineon
>>
>> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
>> ---
>>
>>  drivers/char/tpm/tpm_i2c_infineon.c | 21 +++------------------
>>  1 file changed, 3 insertions(+), 18 deletions(-)
>>
>> diff --git a/drivers/char/tpm/tpm_i2c_infineon.c b/drivers/char/tpm/tpm_i2c_infineon.c
>> index 79d6bbb58e39..005c38879b2e 100644
>> --- a/drivers/char/tpm/tpm_i2c_infineon.c
>> +++ b/drivers/char/tpm/tpm_i2c_infineon.c
>> @@ -675,24 +675,9 @@ MODULE_DEVICE_TABLE(i2c, tpm_tis_i2c_table);
>>  
>>  #ifdef CONFIG_OF
>>  static const struct of_device_id tpm_tis_i2c_of_match[] = {
>> -	{
>> -		.name = "tpm_i2c_infineon",
>> -		.type = "tpm",
>> -		.compatible = "infineon,tpm_i2c_infineon",
>> -		.data = (void *)0
>> -	},
>> -	{
>> -		.name = "slb9635tt",
>> -		.type = "tpm",
>> -		.compatible = "infineon,slb9635tt",
>> -		.data = (void *)0
>> -	},
>> -	{
>> -		.name = "slb9645tt",
>> -		.type = "tpm",
>> -		.compatible = "infineon,slb9645tt",
>> -		.data = (void *)1
>> -	},
>> +	{ .compatible = "infineon,tpm_i2c_infineon", .data = (void *)0 },
>> +	{ .compatible = "infineon,slb9635tt", .data = (void *)0 },
>> +	{ .compatible = "infineon,slb9645tt", .data = (void *)1 },
>>  	{},
>>  };
>>  MODULE_DEVICE_TABLE(of, tpm_tis_i2c_of_match);
>> -- 
>> 2.14.3
>>
> 
> What does the "data" field mean?
>

The field is used to store data that's associated with an entry in the OF table.
It's used by drivers to have per device configuration data to avoid the need to
have different drivers for similar devices.

Although it seems that's not used by this particular driver, so they might be
removed on a follow-up patch. Hard to say that the 0 and 1 magic numbers mean.

As an example, the tpm_i2c_nuvoton driver store in the .data field whether a
TPM device is a 2.0 or 1.2:

static const struct of_device_id i2c_nuvoton_of_match[] = {
	{.compatible = "nuvoton,npct501"},
	{.compatible = "winbond,wpct301"},
	{.compatible = "nuvoton,npct601", .data = OF_IS_TPM2},
	{},
};

static int i2c_nuvoton_probe(struct i2c_client *client,
			     const struct i2c_device_id *id)
{
...
	if (dev->of_node) {
		const struct of_device_id *of_id;

		of_id = of_match_device(dev->driver->of_match_table, dev);
		if (of_id && of_id->data == OF_IS_TPM2)
			chip->flags |= TPM_CHIP_FLAG_TPM2;
	} else
		if (id->driver_data == I2C_IS_TPM2)
			chip->flags |= TPM_CHIP_FLAG_TPM2;
...
}

> /Jarkko
> 

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

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