Re: Data corruption on serial interface under load

[Andy Shevchenko]

On Fri, Feb 5, 2016 at 1:15 AM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Thu, Feb 04, 2016 at 08:55:48PM +0200, Andy Shevchenko wrote:
>> Hi!
>>
>> Today I observed interesting bug / feature of uart layer in the kernel.
>> I do have a setup which connects two identical devices by serial line.
>> I run data transferring in one direction and got data corruption on
>> receiver side (in uart layer, not the driver).
>>
>> Here is the dump from test suite and real data from 8250 registers:
>>
>> === 8< ===
>>
>> Needed 16 reads 0 writes Oh oh, inconsistency at pos 1 (0x1).
>>
>> Original sample:
>> 00000000: 7f 45 4c 46 01 01 01 00  00 00 00 00 00 00 00 00   .ELF............
>> 00000010: 02 00 03 00 01 00 00 00  19 8d 04 08 34 00 00 00   ............4...
>> 00000020: 2c f2 00 00 00 00 00 00  34 00 20 00 04 00 28 00   ,.......4. ...(.
>>
>> Received sample:
>> 00000000: 7f 00 45 00 4c 00 46 00  01 00 01 00 01 00 00 00   ..E.L.F.........
>> 00000010: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
>> 00000020: 02 00 00 00 03 00 00 00  01 00 00 00 00 19 8d 04   ................
>> loops 1 / 1
>>
>> cts: 0 dsr: 0 rng: 0 dcd: 0 rx: 53434 tx: 0 frame 0 ovr 34201 par: 0
>> brk: 0 buf_ovrr: 0
>>
>> === 8< ===
>>
>> R 356.360109 IIR 0xc4
>> R 356.360114 LSR 0x63
>> R 356.360119 RX 0x7f
>
> I think the obvious question here is: why is your serial port reporting
> overrun errors in loopback mode.
>
> If you have no flow control, I suspect this is likely to happen: if we
> try to fill the Tx FIFO, we won't be servicing the port trying to receive
> characters.
>
> So if (eg) the port already contains 12 characters in the RX FIFO, and
> we load up a full complement of characters into the TX FIFO, the port
> will transmit them to the RX side.  As we will not be reading the RX
> side (as we're busy loading the TX side), if we fill the RX FIFO, you'll
> then get overruns.
>
> Even so, with a dumb 8250 based UART, there's no hardware assisted flow
> control, so it's never going to be particularly reliable.  More modern
> UARTs have realised this, and have implemented hardware (and software)
> flow control mechanisms in hardware to reduce the chances of overruns.
>

Yeah, above makes sense to me, but that is another issue I'm
investigating. The issue I complained about is additional '\0'
characters (seems uart_insert_char() does this).


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-- 
With Best Regards,
Andy Shevchenko