USART_CR3_DMAR
cr3 |= USART_CR3_DMAR;
cr3 &= ~USART_CR3_DMAR;
stm32_usart_clr_bits(port, ofs->cr3, USART_CR3_DMAR);
stm32_usart_set_bits(port, ofs->cr3, USART_CR3_DMAR);