[PATCH v2 08/13] usb: xhci: simplify Isochronous Scheduling Threshold handling

[Niklas Neronin <niklas.neronin@linux.intel.com>]

The IST is represented by bits 2:0, with bit 3 indicating the unit of
measurement, Frames or Microframes. Introduce xhci_ist_microframes(),
which returns the IST value in Microframes, simplifying the code and
reducing duplication.

Improve documentation in xhci-caps.h to clarify the IST register specifics,
including the unit conversion details. These change removes the need to
explain it each time the IST values is retrieved.

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
---
v2:
 * Change xhci_ist_in_microseconds() argument to 'xhci'.
 * Rename xhci_ist_in_microseconds() to xhci_ist_microframes().

 drivers/usb/host/xhci-caps.h |  9 ++++++++-
 drivers/usb/host/xhci-ring.c | 26 ++++++++++++--------------
 2 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/drivers/usb/host/xhci-caps.h b/drivers/usb/host/xhci-caps.h
index 8a435786f950..e772d5f30d36 100644
--- a/drivers/usb/host/xhci-caps.h
+++ b/drivers/usb/host/xhci-caps.h
@@ -24,8 +24,15 @@
 /*
  * bits 3:0 - Isochronous Scheduling Threshold, frames or uframes that SW
  * needs to queue transactions ahead of the HW to meet periodic deadlines.
+ * - Bits 2:0: Threshold value
+ * - Bit 3: Unit indicator
+ *   - '1': Threshold in Frames
+ *   - '0': Threshold in Microframes (uframes)
+ * Note: 1 Frame = 8 Microframes
+ * xHCI specification section 5.3.4.
  */
-#define HCS_IST(p)		(((p) >> 0) & 0xf)
+#define HCS_IST_VALUE(p)	((p) & 0x7)
+#define HCS_IST_UNIT(p)		((p) & (1 << 3))
 /* bits 7:4 - Event Ring Segment Table Max, 2^(n) */
 #define HCS_ERST_MAX(p)		(((p) >> 4) & 0xf)
 /* bits 20:8 - Rsvd */
diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 5f46661c8e6b..6ea0ae27617e 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -3983,6 +3983,16 @@ static unsigned int xhci_get_last_burst_packet_count(struct xhci_hcd *xhci,
 	return total_packet_count - 1;
 }
 
+/* Returns the Isochronous Scheduling Threshold in Microframes. 1 Frame is 8 Microframes. */
+static int xhci_ist_microframes(struct xhci_hcd *xhci)
+{
+	int ist = HCS_IST_VALUE(xhci->hcs_params2);
+
+	if (HCS_IST_UNIT(xhci->hcs_params2))
+		ist *= 8;
+	return ist;
+}
+
 /*
  * Calculates Frame ID field of the isochronous TRB identifies the
  * target frame that the Interval associated with this Isochronous
@@ -4002,17 +4012,7 @@ static int xhci_get_isoc_frame_id(struct xhci_hcd *xhci,
 	else
 		start_frame = (urb->start_frame + index * urb->interval) >> 3;
 
-	/* Isochronous Scheduling Threshold (IST, bits 0~3 in HCSPARAMS2):
-	 *
-	 * If bit [3] of IST is cleared to '0', software can add a TRB no
-	 * later than IST[2:0] Microframes before that TRB is scheduled to
-	 * be executed.
-	 * If bit [3] of IST is set to '1', software can add a TRB no later
-	 * than IST[2:0] Frames before that TRB is scheduled to be executed.
-	 */
-	ist = HCS_IST(xhci->hcs_params2) & 0x7;
-	if (HCS_IST(xhci->hcs_params2) & (1 << 3))
-		ist <<= 3;
+	ist = xhci_ist_microframes(xhci);
 
 	/* Software shall not schedule an Isoch TD with a Frame ID value that
 	 * is less than the Start Frame ID or greater than the End Frame ID,
@@ -4333,9 +4333,7 @@ int xhci_queue_isoc_tx_prepare(struct xhci_hcd *xhci, gfp_t mem_flags,
 	 * Round up to the next frame and consider the time before trb really
 	 * gets scheduled by hardare.
 	 */
-	ist = HCS_IST(xhci->hcs_params2) & 0x7;
-	if (HCS_IST(xhci->hcs_params2) & (1 << 3))
-		ist <<= 3;
+	ist = xhci_ist_microframes(xhci);
 	start_frame += ist + XHCI_CFC_DELAY;
 	start_frame = roundup(start_frame, 8);
 
-- 
2.50.1