Avoid I2C timing computation when input clock is unchanged

With U8g2 library, I2C clock is set for each transmission.
With some I2C harwdware version,
timings are then computed for each time.
This cause very slow display on LCD.

Author: ABOSTM

cores/arduino/stm32/twi.c

@@ -62,9 +62,6 @@ extern "C" {
 #ifndef I2C_VALID_TIMING_NBR
 #define I2C_VALID_TIMING_NBR          8U
 #endif
-#define I2C_SPEED_FREQ_STANDARD       0U /* 100 kHz */
-#define I2C_SPEED_FREQ_FAST           1U /* 400 kHz */
-#define I2C_SPEED_FREQ_FAST_PLUS      2U /* 1 MHz */
 #define I2C_ANALOG_FILTER_DELAY_MIN  50U /* ns */
 #ifndef I2C_ANALOG_FILTER_DELAY_MAX
 #define I2C_ANALOG_FILTER_DELAY_MAX 260U /* ns */
@@ -82,6 +79,22 @@ extern "C" {
 #define I2C_SCLL_MAX                256U
 #define SEC2NSEC            1000000000UL
 
+typedef enum {
+  I2C_SPEED_FREQ_STANDARD,  /* 100 kHz */
+  I2C_SPEED_FREQ_FAST,      /* 400 kHz */
+  I2C_SPEED_FREQ_FAST_PLUS, /* 1 MHz */
+  I2C_SPEED_FREQ_NUMBER     /* Must be the last entry */
+} i2c_speed_freq_t;
+
+#ifdef I2C_TIMING
+typedef struct {
+  uint32_t input_clock;      /* I2C Input clock */
+  uint32_t timing;           /* I2C timing corresponding to Input clock */
+} I2C_timing_t;
+
+static I2C_timing_t I2C_ClockTiming[I2C_SPEED_FREQ_NUMBER] = {0};
+#endif
+
 typedef struct {
   uint32_t freq;      /* Frequency in Hz */
   uint32_t freq_min;  /* Minimum frequency in Hz */
@@ -354,6 +367,16 @@ static uint32_t i2c_computeTiming(uint32_t clkSrcFreq, uint32_t i2c_speed)
   uint8_t presc, scldel, sdadel;
   uint32_t tafdel_min, tafdel_max;
 
+  if (i2c_speed > I2C_SPEED_FREQ_NUMBER) {
+    return ret;
+  }
+  /* Don't compute timing if already available value for the requested speed with the same I2C input frequency */
+  if ((I2C_ClockTiming[i2c_speed].input_clock == clkSrcFreq) && (I2C_ClockTiming[i2c_speed].timing != 0U)) {
+    return I2C_ClockTiming[i2c_speed].timing;
+  }
+
+  I2C_ClockTiming[i2c_speed].input_clock = clkSrcFreq;  // Save the I2C input clock for which we will save timing
+
   ti2cclk = (SEC2NSEC + (clkSrcFreq / 2U)) / clkSrcFreq;
   ti2cspeed = (SEC2NSEC + (I2C_Charac[i2c_speed].freq / 2U)) / I2C_Charac[i2c_speed].freq;
 
@@ -437,6 +460,8 @@ static uint32_t i2c_computeTiming(uint32_t clkSrcFreq, uint32_t i2c_speed)
                               ((sclh & 0xFFU) << 8) | \
                               ((scll & 0xFFU) << 0);
                         prev_presc = presc;
+                        /* Save I2C Timing found for further reuse (and avoid to compute again) */
+                        I2C_ClockTiming[i2c_speed].timing = ret;
                       }
                     }
                   }
@@ -736,7 +761,6 @@ i2c_status_e i2c_master_write(i2c_t *obj, uint8_t dev_address,
        Master restarts communication */
   } while (((HAL_I2C_GetError(&(obj->handle)) & HAL_I2C_ERROR_AF) == HAL_I2C_ERROR_AF)
            && (delta < I2C_TIMEOUT_TICK));
-
   return ret;
 }