Re: [PATCH] tty: serial: Use fifo in 8250 console driver

[Jiri Slaby <jirislaby@kernel.org>]

On 25. 01. 22, 9:39, Jon Hunter wrote:
> 
> On 29/10/2021 21:14, wander@redhat.com wrote:
>> From: Wander Lairson Costa <wander@redhat.com>
>>
>> Note: I am using a small test app + driver located at [0] for the
>> problem description. serco is a driver whose write function dispatches
>> to the serial controller. sertest is a user-mode app that writes n bytes
>> to the serial console using the serco driver.
>>
>> While investigating a bug in the RHEL kernel, I noticed that the serial
>> console throughput is way below the configured speed of 115200 bps in
>> a HP Proliant DL380 Gen9. I was expecting something above 10KB/s, but
>> I got 2.5KB/s.
>>
>> $ time ./sertest -n 2500 /tmp/serco
>>
>> real    0m0.997s
>> user    0m0.000s
>> sys     0m0.997s
>>
>> With the help of the function tracer, I then noticed the serial
>> controller was taking around 410us seconds to dispatch one single byte:
>>
>> $ trace-cmd record -p function_graph -g serial8250_console_write \
>>     ./sertest -n 1 /tmp/serco
>>
>> $ trace-cmd report
>>
>>              |  serial8250_console_write() {
>>   0.384 us   |    _raw_spin_lock_irqsave();
>>   1.836 us   |    io_serial_in();
>>   1.667 us   |    io_serial_out();
>>              |    uart_console_write() {
>>              |      serial8250_console_putchar() {
>>              |        wait_for_xmitr() {
>>   1.870 us   |          io_serial_in();
>>   2.238 us   |        }
>>   1.737 us   |        io_serial_out();
>>   4.318 us   |      }
>>   4.675 us   |    }
>>              |    wait_for_xmitr() {
>>   1.635 us   |      io_serial_in();
>>              |      __const_udelay() {
>>   1.125 us   |        delay_tsc();
>>   1.429 us   |      }
>> ...
>> ...
>> ...
>>   1.683 us   |      io_serial_in();
>>              |      __const_udelay() {
>>   1.248 us   |        delay_tsc();
>>   1.486 us   |      }
>>   1.671 us   |      io_serial_in();
>>   411.342 us |    }
>>
>> In another machine, I measured a throughput of 11.5KB/s, with the serial
>> controller taking between 80-90us to send each byte. That matches the
>> expected throughput for a configuration of 115200 bps.
>>
>> This patch changes the serial8250_console_write to use the 16550 fifo
>> if available. In my benchmarks I got around 25% improvement in the slow
>> machine, and no performance penalty in the fast machine.
>>
>> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> 
> 
> On the current mainline and -next branches, I have noticed that the
> serial output on many of our Tegra boards is corrupted and so
> parsing the serial output is failing.
> 
> Before this change the serial console would appear as follows ...
> 
> [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x411fd071]
> [    0.000000] Linux version 5.16.0-rc6-00091-gadbfddc757ae 
> (jonathanh@jonathanh-vm-01) (aarch64-linux-gnu-gcc (Linaro GCC 
> 6.4-2017.08) 6.4.1 20170707, GNU ld (Linaro_Binutils-2017.08) 
> 2.27.0.20161019) #15 SMP PREEMPT Tue Jan 25 00:15:25 PST 2022
> [    0.000000] Machine model: NVIDIA Jetson TX1 Developer Kit
> 
> And now I see ...
> 
> [    0.000000] Booting Linux on physicalfd071]
> [    0.000000] Linux version 5.16.0-rc6-athanh@j-linux-g017.08) 
> Linaro_B20161019n 25 00:[    0.000000] Machine model: NVIDIA Jet[    
> 0.000000] efi: UEFI not found.
> [    0.000000] NUMA: No NUMA configurati[    0.000000] NUMA: Faking a 
> node at [m00000001[    0.000000] NUMA: NODE_DATA [mem 0x17[    0.000000] 
> Zone ranges:
> 
> Bisecting is pointing to this commit. Let me know if there are any
> tests I can run. Otherwise we may need to disable this at least
> for Tegra.


The test is bogus:
         use_fifo = (up->capabilities & UART_CAP_FIFO) &&
                 port->fifosize > 1 &&
                 (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO)

FCR is write only. Reading it, one gets IIR contents.

regards,
-- 
js
suse labs