[PATCH v2] console: usb: kbd: Limit poll frequency to improve performance

[Thomas Watson]

Using the XHCI driver, the function `usb_kbd_poll_for_event` takes
30-40ms to run. The exact time is dependent on the polling interval the
keyboard requests in its descriptor, and likely cannot be significantly
reduced without major rework to the XHCI driver.

The U-Boot EFI console service sets a timer to poll the keyboard every 5
microseconds, and this timer is checked every time a block is read off
disk. The net effect is that, on my system, loading a ~40MiB kernel and
initrd takes about 62 seconds with a slower keyboard and 53 seconds
with a faster one, with the vast majority of the time spent polling the
keyboard.

To solve this problem, this patch adds a 20ms delay between consecutive
calls to `usb_kbd_poll_for_event`. This is sufficient to reduce the
total loading time to under half a second for both keyboards, and does
not impact the perceived keystroke latency.

Signed-off-by: Thomas Watson <twatson52 at icloud.com>
---
This revision wraps the comment at 80 characters and also should not
have been corrupted by my e-mail client.

 common/usb_kbd.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/common/usb_kbd.c b/common/usb_kbd.c
index afad260d3d..5408c821b4 100644
--- a/common/usb_kbd.c
+++ b/common/usb_kbd.c
@@ -118,7 +118,7 @@ struct usb_kbd_pdata {
 extern int __maybe_unused net_busy_flag;
 
 /* The period of time between two calls of usb_kbd_testc(). */
-static unsigned long __maybe_unused kbd_testc_tms;
+static unsigned long kbd_testc_tms;
 
 /* Puts character in the queue and sets up the in and out pointer. */
 static void usb_kbd_put_queue(struct usb_kbd_pdata *data, u8 c)
@@ -394,21 +394,31 @@ static int usb_kbd_testc(struct stdio_dev *sdev)
 	struct usb_device *usb_kbd_dev;
 	struct usb_kbd_pdata *data;
 
+	/*
+	 * Polling the keyboard for an event can take dozens of milliseconds.
+	 * Add a delay between polls to avoid blocking activity which polls
+	 * rapidly, like the UEFI console timer.
+	 */
+	unsigned long poll_delay = CONFIG_SYS_HZ / 50;
+
 #ifdef CONFIG_CMD_NET
 	/*
 	 * If net_busy_flag is 1, NET transfer is running,
 	 * then we check key-pressed every second (first check may be
 	 * less than 1 second) to improve TFTP booting performance.
 	 */
-	if (net_busy_flag && (get_timer(kbd_testc_tms) < CONFIG_SYS_HZ))
-		return 0;
-	kbd_testc_tms = get_timer(0);
+	if (net_busy_flag)
+		poll_delay = CONFIG_SYS_HZ;
 #endif
+
 	dev = stdio_get_by_name(sdev->name);
 	usb_kbd_dev = (struct usb_device *)dev->priv;
 	data = usb_kbd_dev->privptr;
 
-	usb_kbd_poll_for_event(usb_kbd_dev);
+	if (get_timer(kbd_testc_tms) >= poll_delay) {
+		usb_kbd_poll_for_event(usb_kbd_dev);
+		kbd_testc_tms = get_timer(0);
+	}
 
 	return !(data->usb_in_pointer == data->usb_out_pointer);
 }
-- 
2.31.1