rgb_lcd: support fractional clock divisor

components/hal/lcd_hal.c

@@ -1,13 +1,65 @@
 /*
- * SPDX-FileCopyrightText: 2021 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2021-2022 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
 
 #include "hal/lcd_hal.h"
 #include "hal/lcd_ll.h"
+#include "hal/log.h"
 
 void lcd_hal_init(lcd_hal_context_t *hal, int id)
 {
     hal->dev = LCD_LL_GET_HW(id);
 }
+
+/**
+ * @brief helper function, calculate the Greatest Common Divisor
+ * @note gcd(a, b) = gcd(b, a % b)
+ * @param a bigger value
+ * @param b smaller value
+ * @return result of gcd(a, b)
+ */
+static inline uint32_t _gcd(uint32_t a, uint32_t b)
+{
+    uint32_t c = a % b;
+    while (c != 0) {
+        a = b;
+        b = c;
+        c = a % b;
+    }
+    return b;
+}
+
+uint32_t lcd_hal_cal_pclk_freq(lcd_hal_context_t *hal, uint32_t src_freq_hz, uint32_t expect_pclk_freq_hz)
+{
+    // lcd_clk = module_clock_src / (n + b / a)
+    // pixel_clk = lcd_clk / mo
+    uint32_t mo = src_freq_hz / expect_pclk_freq_hz / LCD_LL_CLK_FRAC_DIV_N_MAX + 1;
+    uint32_t n = src_freq_hz / expect_pclk_freq_hz / mo;
+    uint32_t a = 0;
+    uint32_t b = 0;
+    // delta_hz / expect_pclk_freq_hz <==> b / a
+    uint32_t delta_hz = src_freq_hz - expect_pclk_freq_hz * mo * n;
+    // fractional divider
+    if (delta_hz) {
+        uint32_t gcd = _gcd(expect_pclk_freq_hz, delta_hz);
+        a = expect_pclk_freq_hz / gcd;
+        b = delta_hz / gcd;
+        // normalize div_a and div_b
+        uint32_t d = a / LCD_LL_CLK_FRAC_DIV_AB_MAX + 1;
+        a /= d;
+        b /= d;
+    }
+
+    HAL_LOGD("lcd_hal", "n=%d,a=%d,b=%d,mo=%d", n, a, b, mo);
+
+    lcd_ll_set_group_clock_coeff(hal->dev, n, a, b);
+    lcd_ll_set_pixel_clock_prescale(hal->dev, mo);
+
+    if (delta_hz) {
+        return ((uint64_t)src_freq_hz * a) / (n * a + b) / mo;
+    } else {
+        return src_freq_hz / n / mo;
+    }
+}