firewire: add kernel API to access CYCLE_TIME register

1394 OHCI specification defined Isochronous Cycle Timer Register to get
value of CYCLE_TIME register defined by IEEE 1394 for CSR architecture
defined by ISO/IEC 13213. Unit driver can calculate packet time by
compute with the value of CYCLE_TIME and timeStamp field in descriptor
of each isochronous and asynchronous context. The resolution of CYCLE_TIME
is 49.576 MHz, while the one of timeStamp is 8,000 Hz.

Current implementation of Linux FireWire subsystem allows the driver to
get the value of CYCLE_TIMER CSR register by transaction service. The
transaction service has overhead in regard of access to MMIO register.

This commit adds kernel API for unit driver to access the register
directly.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20220405072221.226217-3-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>

drivers/firewire/core-card.c

@@ -702,3 +702,31 @@ void fw_core_remove_card(struct fw_card *card)
 	WARN_ON(!list_empty(&card->transaction_list));
 }
 EXPORT_SYMBOL(fw_core_remove_card);
+
+/**
+ * fw_card_read_cycle_time: read from Isochronous Cycle Timer Register of 1394 OHCI in MMIO region
+ *			    for controller card.
+ * @card: The instance of card for 1394 OHCI controller.
+ * @cycle_time: The mutual reference to value of cycle time for the read operation.
+ *
+ * Read value from Isochronous Cycle Timer Register of 1394 OHCI in MMIO region for the given
+ * controller card. This function accesses the region without any lock primitives or IRQ mask.
+ * When returning successfully, the content of @value argument has value aligned to host endianness,
+ * formetted by CYCLE_TIME CSR Register of IEEE 1394 std.
+ *
+ * Context: Any context.
+ * Return:
+ * * 0 - Read successfully.
+ * * -ENODEV - The controller is unavailable due to being removed or unbound.
+ */
+int fw_card_read_cycle_time(struct fw_card *card, u32 *cycle_time)
+{
+	if (card->driver->read_csr == dummy_read_csr)
+		return -ENODEV;
+
+	// It's possible to switch to dummy driver between the above and the below. This is the best
+	// effort to return -ENODEV.
+	*cycle_time = card->driver->read_csr(card, CSR_CYCLE_TIME);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fw_card_read_cycle_time);

drivers/firewire/core-cdev.c

@@ -1216,7 +1216,9 @@ static int ioctl_get_cycle_timer2(struct client *client, union ioctl_arg *arg)
 
 	local_irq_disable();
 
-	cycle_time = card->driver->read_csr(card, CSR_CYCLE_TIME);
+	ret = fw_card_read_cycle_time(card, &cycle_time);
+	if (ret < 0)
+		goto end;
 
 	switch (a->clk_id) {
 	case CLOCK_REALTIME:      ktime_get_real_ts64(&ts);	break;
@@ -1225,7 +1227,7 @@ static int ioctl_get_cycle_timer2(struct client *client, union ioctl_arg *arg)
 	default:
 		ret = -EINVAL;
 	}
-
+end:
 	local_irq_enable();
 
 	a->tv_sec      = ts.tv_sec;

include/linux/firewire.h

@@ -150,6 +150,8 @@ static inline void fw_card_put(struct fw_card *card)
 	kref_put(&card->kref, fw_card_release);
 }
 
+int fw_card_read_cycle_time(struct fw_card *card, u32 *cycle_time);
+
 struct fw_attribute_group {
 	struct attribute_group *groups[2];
 	struct attribute_group group;