Re: [PATCH v2 1/4] tpm: Cache TPM2 start method

[Jarkko Sakkinen <jarkko@kernel.org>]

On Sat, Nov 14, 2026 at 07:34:31PM +0000, Siddharth Chintamaneni wrote:
> From: Jarkko Sakkinen <jarkko@kernel.org>
> 
> Both tpm_tis and tpm_crb retrieve the start method from the ACPI TPM2
> table during device probe, and tpm_crb_ffa also needs it before chip
> allocation.
> 
> Cache the start method in tpm2_acpi_start_method during tpm_init() and
> export it for the leaf drivers, defaulting to ACPI_TPM2_NOT_ALLOWED when
> the table is absent or invalid.
> 
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> Co-developed-by: Siddharth Chintamaneni <sidchintamaneni@gmail.com>
> Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@gmail.com>
> ---
>  drivers/char/tpm/tpm-interface.c | 21 +++++++++++++++++++++
>  drivers/char/tpm/tpm.h           |  1 +
>  2 files changed, 22 insertions(+)
> 
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index e0385b16616b..7b7da79bee7c 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -534,8 +534,17 @@ int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
>  }
>  EXPORT_SYMBOL_GPL(tpm_get_random);
>  
> +#ifdef CONFIG_ACPI
> +u32 tpm2_acpi_start_method = ACPI_TPM2_NOT_ALLOWED;
> +EXPORT_SYMBOL_GPL(tpm2_acpi_start_method);
> +#endif
> +
>  static int __init tpm_init(void)
>  {
> +#ifdef CONFIG_ACPI
> +	struct acpi_table_tpm2 *tbl;
> +	acpi_status status;
> +#endif
>  	int rc;
>  
>  	rc = class_register(&tpm_class);
> @@ -562,6 +571,18 @@ static int __init tpm_init(void)
>  		goto out_unreg_chrdev;
>  	}
>  
> +#ifdef CONFIG_ACPI
> +	if (!acpi_disabled) {
> +		status = acpi_get_table(ACPI_SIG_TPM2, 1,
> +					(struct acpi_table_header **)&tbl);
> +		if (ACPI_SUCCESS(status)) {
> +			if (tbl->header.length >= sizeof(*tbl))
> +				tpm2_acpi_start_method = tbl->start_method;
> +			acpi_put_table((struct acpi_table_header *)tbl);

I would add right after acpi_put_table:

	pr_info("tpm: TPM2 ACPI start method: 0%02x\n", tpm2_acpi_start_method);

It covers all three possible conditions:

1. No TPM2: nothing gets printed.
2. Has TPM2 and start method: start method gets printed.
3. Has TPM2 but no start emthod: 0x00 gets printed.

I hate fostering klog messages as we have acquired great tracing
capabilities over the years but here I think having this is even
"sysadmin-useful".

Br, Jarkko