tpm_crb_ffa: handle tpm busy return code

Platforms supporting direct message request v2 [1] can support secure
partitions that support multiple services. For CRB over FF-A interface,
if the firmware TPM or TPM service [1] shares its Secure Partition (SP)
with another service, message requests may fail with a -EBUSY error.

To handle this, replace the single check and call with a retry loop
that attempts the TPM message send operation until it succeeds or a
configurable timeout is reached. Implement a _try_send_receive function
to do a single send/receive and modify the existing send_receive to
add this retry loop.
The retry mechanism introduces a module parameter (`busy_timeout_ms`,
default: 2000ms) to control how long to keep retrying on -EBUSY
responses. Between retries, the code waits briefly (50-100 microseconds)
to avoid busy-waiting and handling TPM BUSY conditions more gracefully.

The parameter can be modified at run-time as such:
echo 3000 | tee /sys/module/tpm_crb_ffa/parameters/busy_timeout_ms
This changes the timeout from the default 2000ms to 3000ms.

[1] TPM Service Command Response Buffer Interface Over FF-A
https://developer.arm.com/documentation/den0138/latest/

Signed-off-by: Prachotan Bathi <prachotan.bathi@arm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Author: PrachotanReddy

Documentation/admin-guide/kernel-parameters.txt

@@ -7214,6 +7214,14 @@
 			causing a major performance hit, and the space where
 			machines are deployed is by other means guarded.
 
+	tpm_crb_ffa.busy_timeout_ms= [ARM64,TPM]
+			Maximum time in milliseconds to retry sending a message
+			to the TPM service before giving up. This parameter controls
+			how long the system will continue retrying when the TPM
+			service is busy.
+			Format: <unsigned int>
+			Default: 2000 (2 seconds)
+
 	tpm_suspend_pcr=[HW,TPM]
 			Format: integer pcr id
 			Specify that at suspend time, the tpm driver

drivers/char/tpm/tpm_crb_ffa.c

@@ -10,8 +10,16 @@
 #define pr_fmt(fmt) "CRB_FFA: " fmt
 
 #include <linux/arm_ffa.h>
+#include <linux/delay.h>
+#include <linux/moduleparam.h>
 #include "tpm_crb_ffa.h"
 
+static unsigned int busy_timeout_ms = 2000;
+
+module_param(busy_timeout_ms, uint, 0644);
+MODULE_PARM_DESC(busy_timeout_ms,
+		 "Maximum time in ms to retry before giving up on busy");
+
 /* TPM service function status codes */
 #define CRB_FFA_OK			0x05000001
 #define CRB_FFA_OK_RESULTS_RETURNED	0x05000002
@@ -189,17 +197,13 @@ int tpm_crb_ffa_init(void)
 }
 EXPORT_SYMBOL_GPL(tpm_crb_ffa_init);
 
-static int __tpm_crb_ffa_send_receive(unsigned long func_id,
-				      unsigned long a0,
-				      unsigned long a1,
-				      unsigned long a2)
+static int __tpm_crb_ffa_try_send_receive(unsigned long func_id,
+					  unsigned long a0, unsigned long a1,
+					  unsigned long a2)
 {
 	const struct ffa_msg_ops *msg_ops;
 	int ret;
 
-	if (!tpm_crb_ffa)
-		return -ENOENT;
-
 	msg_ops = tpm_crb_ffa->ffa_dev->ops->msg_ops;
 
 	if (ffa_partition_supports_direct_req2_recv(tpm_crb_ffa->ffa_dev)) {
@@ -225,6 +229,33 @@ static int __tpm_crb_ffa_send_receive(unsigned long func_id,
 			ret = tpm_crb_ffa_to_linux_errno(tpm_crb_ffa->direct_msg_data.data1);
 	}
 
+	return ret;
+}
+
+static int __tpm_crb_ffa_send_receive(unsigned long func_id, unsigned long a0,
+				      unsigned long a1, unsigned long a2)
+{
+	ktime_t start, stop;
+	int ret;
+
+	if (!tpm_crb_ffa)
+		return -ENOENT;
+
+	start = ktime_get();
+	stop = ktime_add(start, ms_to_ktime(busy_timeout_ms));
+
+	for (;;) {
+		ret = __tpm_crb_ffa_try_send_receive(func_id, a0, a1, a2);
+		if (ret != -EBUSY)
+			break;
+
+		usleep_range(50, 100);
+		if (ktime_after(ktime_get(), stop)) {
+			dev_warn(&tpm_crb_ffa->ffa_dev->dev,
+				 "Busy retry timed out\n");
+			break;
+		}
+	}
 
 	return ret;
 }