Re: [PATCH v4 2/2] tpm: tpm_crb_ffa: try to probe tpm_crb_ffa when it's built-in

[Mimi Zohar]

On Wed, 2025-06-18 at 11:23 +0100, Yeoreum Yun wrote:
> To generate the boot_aggregate log in the IMA subsystem using TPM PCR values,
> the TPM driver must be built as built-in and must be probed before
> the IMA subsystem is initialized.
> 
> However, when the TPM device operates over the FF-A protocol using the CRB interface,
> probing fails and returns -EPROBE_DEFER
> if the tpm_crb_ffa device — an FF-A device that provides
> the communication interface to the tpm_crb driver — has not yet been probed.
> 
> This issue occurs because both crb_acpi_driver_init() and
> tpm_crb_ffa_driver_init() are registered with device_initcall.
> As a result, crb_acpi_driver_init() may be invoked before
> tpm_crb_ffa_driver_init(), which is responsible for probing the tpm_crb_ffa device.
> 
> When this happens, IMA fails to detect the TPM device and
> logs the following message:
> 
>   | ima: No TPM chip found, activating TPM-bypass!
> 
> Consequently, it cannot generate the boot_aggregate log with
> the PCR values provided by the TPM.
> 
> To resolve this issue, the tpm_crb_ffa_init() function explicitly attempts to
> probe the tpm_crb_ffa by register tpm_crb_ffa driver so that
> when tpm_crb_ffa device is created before tpm_crb_ffa_init(),
> probe the tpm_crb_ffa device in tpm_crb_ffa_init() to finish probe the
> TPM device completely.
> 
> This ensures that the TPM device using CRB over FF-A
> can be successfully probed, even if crb_acpi_driver_init() is called first.
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
> ---
>  drivers/char/tpm/tpm_crb_ffa.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm_crb_ffa.c b/drivers/char/tpm/tpm_crb_ffa.c
> index 4ead61f01299..462fcf610020 100644
> --- a/drivers/char/tpm/tpm_crb_ffa.c
> +++ b/drivers/char/tpm/tpm_crb_ffa.c
> @@ -115,6 +115,7 @@ struct tpm_crb_ffa {
>  };
>  
>  static struct tpm_crb_ffa *tpm_crb_ffa;
> +static struct ffa_driver tpm_crb_ffa_driver;
>  
>  static int tpm_crb_ffa_to_linux_errno(int errno)
>  {
> @@ -168,13 +169,23 @@ static int tpm_crb_ffa_to_linux_errno(int errno)
>   */
>  int tpm_crb_ffa_init(void)
>  {
> +	int ret = 0;
> +
> +	if (!IS_MODULE(CONFIG_TCG_ARM_CRB_FFA)) {
> +		ret = ffa_register(&tpm_crb_ffa_driver);
> +		if (ret) {
> +			tpm_crb_ffa = ERR_PTR(-ENODEV);
> +			return ret;
> +		}
> +	}
> +
>  	if (!tpm_crb_ffa)
> -		return -ENOENT;
> +		ret = -ENOENT;
>  
>  	if (IS_ERR_VALUE(tpm_crb_ffa))
> -		return -ENODEV;
> +		ret = -ENODEV;
>  
> -	return 0;
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(tpm_crb_ffa_init);
>  
> @@ -369,7 +380,9 @@ static struct ffa_driver tpm_crb_ffa_driver = {
>  	.id_table = tpm_crb_ffa_device_id,
>  };
>  
> +#ifdef MODULE
>  module_ffa_driver(tpm_crb_ffa_driver);
> +#endif
>  
>  MODULE_AUTHOR("Arm");
>  MODULE_DESCRIPTION("TPM CRB FFA driver");

LGTM.  Using ifndef/ifdef MODULE is similar to how module_init() works for both
builtin and loadable kernel modules. Except module_init() is on the
device_initcall().

Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>