On 19 April 2017 at 10:25, Jiahuan Zhang <jiahuanzhan...@gmail.com> wrote:
> On 19 April 2017 at 11:15, Peter Maydell <peter.mayd...@linaro.org> wrote:
>> What is happening is that the guest kernel's serial driver
>> has a loop that (simplified) looks like this:
>>
>> do {
>> if (pl011_read(REG_FR) & FR_TXFF)
>> break; /* fifo full, try again later */
>> pl011_write(buffer[x], REG_DR); /* send one byte */
>> x++;
>> } while (x != len);
>>
>> This is a lot of guest CPU instructions (and two callouts
>> to QEMU's device emulation) for every single byte.
>>
> Hi, no, I am not using any kernel driver and I only test the guest to host
> data transfer.
OK, then the equivalent loop is this one:
> /*
> * write_to_uart(): write data to serial port
> */
> void write_to_uart(char* out, uint32_t writeSize){
> int i;
> for(i=0; i<writeSize; i++){
> *UART1 =(unsigned char)(*(out+i));
> }
> }
except that your code is broken because it's not
checking that the FIFO is ready to receive the character
so it will drop data sometimes.
The point is the same -- you're feeding the data to
the UART byte-at-a-time.
thanks
-- PMM
