Re: [PATCH v1 7/8] tpm: tis-i2c: Add more compatible strings

[Guenter Roeck]

On 12/12/23 10:51, Conor Dooley wrote:
> On Tue, Dec 12, 2023 at 10:00:39AM -0800, Guenter Roeck wrote:
> > On Tue, Dec 12, 2023 at 05:15:51PM +0000, Conor Dooley wrote:
> > > On Tue, Dec 12, 2023 at 10:40:03AM -0600, Ninad Palsule wrote:
> > > > From: Joel Stanley <joel@xxxxxxxxx>
> > > >
> > > > The NPCT75x TPM is TIS compatible. It has an I2C and SPI interface.
> > > >
> > > > https://www.nuvoton.com/products/cloud-computing/security/trusted-platform-module-tpm/
> > > >
> > > > Add a compatible string for it, and the generic compatible.
> > > >
> > > > OpenBMC-Staging-Count: 3
> > >
> > > Delete this from every patch that it appears from.
> > >
> > > > Signed-off-by: Joel Stanley <joel@xxxxxxxxx>
> > > > Acked-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > > > Link: https://lore.kernel.org/r/20220928043957.2636877-4-joel@xxxxxxxxx
> > > > Signed-off-by: Ninad Palsule <ninad@xxxxxxxxxxxxx>
> > > > ---
> > > >   drivers/char/tpm/tpm_tis_i2c.c | 2 ++
> > > >   1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/drivers/char/tpm/tpm_tis_i2c.c b/drivers/char/tpm/tpm_tis_i2c.c
> > > > index a897402cc36a..9511c0d50185 100644
> > > > --- a/drivers/char/tpm/tpm_tis_i2c.c
> > > > +++ b/drivers/char/tpm/tpm_tis_i2c.c
> > > > @@ -383,6 +383,8 @@ MODULE_DEVICE_TABLE(i2c, tpm_tis_i2c_id);
> > > >   #ifdef CONFIG_OF
> > > >   static const struct of_device_id of_tis_i2c_match[] = {
> > > >   	{ .compatible = "infineon,slb9673", },
> > > > +	{ .compatible = "nuvoton,npct75x", },
> > > > +	{ .compatible = "tcg,tpm-tis-i2c", },
> > >
> > > What's the point of the generic compatible if you are adding the device
> > > specific ones to the driver anyway?
> >
> > $ git grep infineon,slb9673
> > Documentation/devicetree/bindings/trivial-devices.yaml:          - infineon,slb9673
>
> Hmm, this would then need to be moved into the new schema, out of
> trivial devices.
>
> > drivers/char/tpm/tpm_tis_i2c.c: { .compatible = "infineon,slb9673", },
> > $ git grep nuvoton,npct75x
> > arch/arm/boot/dts/aspeed/aspeed-bmc-ibm-bonnell.dts:            compatible = "nuvoton,npct75x", "tcg,tpm-tis-i2c";
> > arch/arm/boot/dts/aspeed/aspeed-bmc-ibm-everest.dts:            compatible = "nuvoton,npct75x", "tcg,tpm-tis-i2c";
> > $ git grep tcg,tpm-tis-i2c
> > arch/arm/boot/dts/aspeed/aspeed-bmc-ibm-bonnell.dts:            compatible = "nuvoton,npct75x", "tcg,tpm-tis-i2c";
> > arch/arm/boot/dts/aspeed/aspeed-bmc-ibm-everest.dts:            compatible = "nuvoton,npct75x", "tcg,tpm-tis-i2c";
> > arch/arm/boot/dts/aspeed/aspeed-bmc-opp-tacoma.dts:             compatible = "tcg,tpm-tis-i2c";
>
> pog, undocumented compatibles.

Yes, I know, quite annoying. Though, to be fair, a generic "tcg,tpm-tis-i2c"
would make a lot of sense.

Note that Rob had rejected the original addition (into trivial devices)
with the argument that it is not a trivial device
(https://lore.kernel.org/lkml/20220605225610.GA3682221-robh@xxxxxxxxxx/).

> > It looks like at least the generic entry is needed, given that it is quite
> > likely that there is hardware out there using it. Other than that, this
> > makes me wonder: Is there some official guideline describing if and when
> > to use (only) generic devicetree compatible entries and when specific ones
> > may / should / have to be used ? I suspect the answer to your question might
> > simply be "because we did not know better", and it might be helpful to be
> > able to say "please see XXX for details".
>
> To me using generic compatibles is okay when there is another mechanism
> to identify the device. This patch would make more sense if the addition
> of nuvoton,npct75x was omitted and the dt-binding had
>
> properties:
>    compatible:
>      items:
>        - enum:
>            - infineon,slb9673
>            - nuvoton,npct75x
>        - const: tcg,tpm-tis-i2c
>
> And whenever new i2c tpms showed up the device specific compatible was
> added to the bindings and the driver had only* the generic compatible
> static const struct of_device_id of_tis_i2c_match[] = {
> 	{ .compatible = "infineon,slb9673", },
> 	{ .compatible = "tcg,tpm-tis-i2c", },
> };
>
> * well, and the existing one since that cannot be removed.

That would be perfectly fine with me. All I personally care about is to get
"tcg,tpm-tis-i2c" added to the kernel source so I can start testing the
code in qemu.

Thanks,
Guenter