stlinkv3: Use a faster clocksource for both UARTs, add docs

* USB UART is USART6 on APB2 with 108 MHz,
  SWO UART is UART5 on APB1 with 54 MHz,
* Switch both to Sysclk 216 MHz given F723 is flexible,
  to reach higher baudrates with default DIV16 prescaler

src/platforms/stlinkv3/README.md

@@ -28,6 +28,17 @@ The ST firmware checks the Romtable and only allows access to STM32 devices. In
 some situations, Romtable access may also fail on STM32 device and so a debugger
 warm plug will fail. Cold plug should work with any STM32 device.
 
+## Performance
+Stock firmware has three settings: High-performance (192M), Standard frequency (96M), Low-consumption (48M).
+SWD up to 24000 kHz, JTAG up to 21333 kHz (SPI), SWO up to 16 Mbaud, VCP 732-16000000 baud.
+Suffix 7 devices are rated for -40..+125 deg C but at 200 MHz frequency maximum.
+Other mentioned limits from DS11853 are 144M, 168M, 180M for scales 3-2-1 and overdrive on/off.
+
+This firmware always runs at 216 MHz (suffix 6). AHB clock = 216M, APB1 = 54M, APB2 = 108M.
+USART1, USART6 on APB2 (108M Pclk), others (including UART5) on APB1.
+SWO capture is implemented as UART5 Rx DMA. 3375k (4500k with OVER8) is the limit.
+13.5 Mbaud (18M with OVER8) could be derived from Sysclk, 216/N where N>=16 (OVER8: 2*216/N where 24<=N<=31).
+
 ## Building.
 
 As simple as

src/platforms/stlinkv3/platform.c

@@ -230,6 +230,19 @@ void platform_init(void)
  gpio_mode_setup(LED_PORT, GPIO_MODE_OUTPUT, GPIO_PUPD_NONE, LED_PIN);
  gpio_set_output_options(LED_PORT, GPIO_OTYPE_PP, GPIO_OSPEED_2MHZ, LED_PIN);
 
+ /* Switch SWO UART clocksource from Pclk (54M) to Hclk (216M) */
+ const uint8_t uart5_clksel = RCC_DCKCFGR2_UARTxSEL_SYSCLK;
+ uint32_t regval = RCC_DCKCFGR2;
+ regval &= ~(RCC_DCKCFGR2_UARTxSEL_MASK << RCC_DCKCFGR2_UART5SEL_SHIFT);
+ regval |= (uart5_clksel & RCC_DCKCFGR2_UARTxSEL_MASK) << RCC_DCKCFGR2_UART5SEL_SHIFT;
+ RCC_DCKCFGR2 = regval;
+ /* Switch USB UART clocksource from Pclk (108M) to Hclk (216M) */
+ const uint8_t usart6_clksel = RCC_DCKCFGR2_UARTxSEL_SYSCLK;
+ regval = RCC_DCKCFGR2;
+ regval &= ~(RCC_DCKCFGR2_UARTxSEL_MASK << RCC_DCKCFGR2_USART6SEL_SHIFT);
+ regval |= (usart6_clksel & RCC_DCKCFGR2_UARTxSEL_MASK) << RCC_DCKCFGR2_USART6SEL_SHIFT;
+ RCC_DCKCFGR2 = regval;
+
  /* Relocate interrupt vector table here */
  SCB_VTOR = (uintptr_t)&vector_table;