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Date: Thu, 4 Feb 2016 20:55:48 +0200
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Subject: Data corruption on serial interface under load
From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Russell King <linux@arm.linux.org.uk>,
        Peter Hurley <peter@hurleysoftware.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-serial@vger.kernel.org" <linux-serial@vger.kernel.org>
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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
R 356.360124 LSR 0x63
R 356.360128 RX 0x45
R 356.360133 LSR 0x63
R 356.360137 RX 0x4c
R 356.360142 LSR 0x63
R 356.360147 RX 0x46
R 356.360151 LSR 0x63
R 356.360156 RX 0x01
R 356.360160 LSR 0x63
R 356.360165 RX 0x01
R 356.360169 LSR 0x63
R 356.360174 RX 0x01

As we can see the data is corrupted on Linux side. Can we somehow fix
this bug/feature?

-- 
With Best Regards,
Andy Shevchenko