[PATCH v2 04/15] HID: ft260: i2c: reduce driver module loading time

[Michael Zaidman]

The HWMON class I2C client drivers register the address range to scan
during the I2C master module probing. For example, the lm75 module
registers eight bus addresses scanned during the ft260 module loading.

In the case of ft260, the bus is scanned by ft260_i2c_read, but due to
an excessive read completion timeout, it took about 40 seconds for the
ft260 driver to scan the lm75 address range before exiting the probe
function.

Adjust the i2c_read timeout so it depends on the amount of data to
read, reducing the lm75 address range scan time to about 200ms.

The 25 ms per-report budget was checked on a UMFT260EV1A with a 24LC512
EEPROM by shrinking the timeout until reads began to fail: a 60-byte
read still completes at 9 ms and times out at 7 ms, against 5.4 ms of
wire time at 100 kHz, so the fixed USB, HID and scheduling overhead
costs a few milliseconds. At the chip's slowest 60 kHz clock that chunk
needs about 9 ms on the wire and a 180-byte chunk about 27 ms. Scale
the wait by 25 ms per 60-byte HID payload so a last chunk of 61..180
bytes gets 50..75 ms, not the single-report 25 ms budget (reported by
Sashiko).

Signed-off-by: Michael Zaidman <michael.zaidman@xxxxxxxxx>

---
v2: last I2C read chunk timeout from rd_len (reported by Sashiko).
---
 drivers/hid/hid-ft260.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/hid/hid-ft260.c b/drivers/hid/hid-ft260.c
index e24f3151fdaf..f134fab08285 100644
--- a/drivers/hid/hid-ft260.c
+++ b/drivers/hid/hid-ft260.c
@@ -17,6 +17,7 @@
 #include <linux/serial_core.h>
 #include <linux/kfifo.h>
 #include <linux/tty_flip.h>
+#include <linux/math.h>
 #include <linux/minmax.h>
 #include <linux/unaligned.h>
 #include <linux/bits.h>
@@ -49,14 +50,20 @@ MODULE_PARM_DESC(debug, "Toggle FT260 debugging messages");
  * The ft260 input report format defines 62 bytes for the data payload, but
  * when requested 62 bytes, the controller returns 60 and 2 in separate input
  * reports. To achieve better performance with the multi-report read data
- * transfers, we set the maximum read payload length to a multiple of 60.
+ * transfers, we set the maximum read payload length to a multiple of
+ * FT260_RD_ONE_REPORT_MAX.
  * With a 100 kHz I2C clock, one 240 bytes read takes about 1/27 second,
  * which is excessive; On the other hand, some higher layer drivers like at24
  * or optoe limit the i2c reads to 128 bytes. To not block other drivers out
  * of I2C for potentially troublesome amounts of time, we select the maximum
  * read payload length to be 180 bytes.
  */
+#define FT260_RD_ONE_REPORT_MAX 60
 #define FT260_RD_DATA_MAX 180
+
+/* Time in ms to wait for a single HID input report at ONE_REPORT_MAX */
+#define FT260_RD_ONE_REPORT_TO 25
+
 #define FT260_WR_I2C_DATA_MAX 60
 #define FT260_WR_UART_DATA_MAX 62
 #define FT260_GPIOCHIP "ft260_gpio"
@@ -722,8 +729,8 @@ static int ft260_i2c_read(struct ft260_device *dev, u8 addr, u8 *data,
 			  u16 len, u8 flag)
 {
 	u16 rd_len;
-	u16 rd_data_max = 60;
-	int timeout, ret = 0;
+	u16 rd_data_max = FT260_RD_ONE_REPORT_MAX;
+	int timeout, timeout_jiffies, ret = 0;
 	struct ft260_i2c_read_request_report rep;
 	struct hid_device *hdev = dev->hdev;
 	unsigned long irqflags;
@@ -748,6 +755,8 @@ static int ft260_i2c_read(struct ft260_device *dev, u8 addr, u8 *data,
 		} else {
 			rd_len = rd_data_max;
 		}
+		timeout = FT260_RD_ONE_REPORT_TO *
+			  DIV_ROUND_UP(rd_len, FT260_RD_ONE_REPORT_MAX);
 		rd_data_max = FT260_RD_DATA_MAX;
 
 		rep.report = FT260_I2C_READ_REQ;
@@ -772,8 +781,8 @@ static int ft260_i2c_read(struct ft260_device *dev, u8 addr, u8 *data,
 			goto ft260_i2c_read_exit;
 		}
 
-		timeout = msecs_to_jiffies(5000);
-		if (!wait_for_completion_timeout(&dev->wait, timeout)) {
+		timeout_jiffies = msecs_to_jiffies(timeout);
+		if (!wait_for_completion_timeout(&dev->wait, timeout_jiffies)) {
 			ret = -ETIMEDOUT;
 			ft260_i2c_reset(hdev);
 			goto ft260_i2c_read_exit;
-- 
2.43.0