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

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 <redacted>
---
 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");