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path: root/drivers/usb/serial/ch341.c
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Diffstat (limited to 'drivers/usb/serial/ch341.c')
-rw-r--r--drivers/usb/serial/ch341.c97
1 files changed, 75 insertions, 22 deletions
diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index 3bb1fff02bed..df582fe855f0 100644
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -48,12 +48,6 @@
#define CH341_BIT_DCD 0x08
#define CH341_BITS_MODEM_STAT 0x0f /* all bits */
-/*******************************/
-/* baudrate calculation factor */
-/*******************************/
-#define CH341_BAUDBASE_FACTOR 1532620800
-#define CH341_BAUDBASE_DIVMAX 3
-
/* Break support - the information used to implement this was gleaned from
* the Net/FreeBSD uchcom.c driver by Takanori Watanabe. Domo arigato.
*/
@@ -144,37 +138,96 @@ static int ch341_control_in(struct usb_device *dev,
return 0;
}
+#define CH341_CLKRATE 48000000
+#define CH341_CLK_DIV(ps, fact) (1 << (12 - 3 * (ps) - (fact)))
+#define CH341_MIN_RATE(ps) (CH341_CLKRATE / (CH341_CLK_DIV((ps), 1) * 512))
+
+static const speed_t ch341_min_rates[] = {
+ CH341_MIN_RATE(0),
+ CH341_MIN_RATE(1),
+ CH341_MIN_RATE(2),
+ CH341_MIN_RATE(3),
+};
+
+/*
+ * The device line speed is given by the following equation:
+ *
+ * baudrate = 48000000 / (2^(12 - 3 * ps - fact) * div), where
+ *
+ * 0 <= ps <= 3,
+ * 0 <= fact <= 1,
+ * 2 <= div <= 256 if fact = 0, or
+ * 9 <= div <= 256 if fact = 1
+ */
+static int ch341_get_divisor(speed_t speed)
+{
+ unsigned int fact, div, clk_div;
+ int ps;
+
+ /*
+ * Clamp to supported range, this makes the (ps < 0) and (div < 2)
+ * sanity checks below redundant.
+ */
+ speed = clamp(speed, 46U, 3000000U);
+
+ /*
+ * Start with highest possible base clock (fact = 1) that will give a
+ * divisor strictly less than 512.
+ */
+ fact = 1;
+ for (ps = 3; ps >= 0; ps--) {
+ if (speed > ch341_min_rates[ps])
+ break;
+ }
+
+ if (ps < 0)
+ return -EINVAL;
+
+ /* Determine corresponding divisor, rounding down. */
+ clk_div = CH341_CLK_DIV(ps, fact);
+ div = CH341_CLKRATE / (clk_div * speed);
+
+ /* Halve base clock (fact = 0) if required. */
+ if (div < 9 || div > 255) {
+ div /= 2;
+ clk_div *= 2;
+ fact = 0;
+ }
+
+ if (div < 2)
+ return -EINVAL;
+
+ /*
+ * Pick next divisor if resulting rate is closer to the requested one,
+ * scale up to avoid rounding errors on low rates.
+ */
+ if (16 * CH341_CLKRATE / (clk_div * div) - 16 * speed >=
+ 16 * speed - 16 * CH341_CLKRATE / (clk_div * (div + 1)))
+ div++;
+
+ return (0x100 - div) << 8 | fact << 2 | ps;
+}
+
static int ch341_set_baudrate_lcr(struct usb_device *dev,
struct ch341_private *priv, u8 lcr)
{
- short a;
+ int val;
int r;
- unsigned long factor;
- short divisor;
if (!priv->baud_rate)
return -EINVAL;
- factor = (CH341_BAUDBASE_FACTOR / priv->baud_rate);
- divisor = CH341_BAUDBASE_DIVMAX;
-
- while ((factor > 0xfff0) && divisor) {
- factor >>= 3;
- divisor--;
- }
- if (factor > 0xfff0)
+ val = ch341_get_divisor(priv->baud_rate);
+ if (val < 0)
return -EINVAL;
- factor = 0x10000 - factor;
- a = (factor & 0xff00) | divisor;
-
/*
* CH341A buffers data until a full endpoint-size packet (32 bytes)
* has been received unless bit 7 is set.
*/
- a |= BIT(7);
+ val |= BIT(7);
- r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, a);
+ r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, val);
if (r)
return r;