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Message-ID: <PR3P174MB0112D073D0E5E080FAAE8510846E9@PR3P174MB0112.EURP174.PROD.OUTLOOK.COM>
Date: Tue, 7 Dec 2021 16:53:12 +0000
From: "Modilaynen, Pavel" <pavel.modilaynen@...vocars.com>
To: "mkl@...gutronix.de" <mkl@...gutronix.de>
CC: "drew@...gleboard.org" <drew@...gleboard.org>,
"linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
"menschel.p@...teo.de" <menschel.p@...teo.de>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"will@...china.cc" <will@...china.cc>
Subject: Re: [net-next 6/6] can: mcp251xfd: mcp251xfd_regmap_crc_read(): work
around broken CRC on TBC register
Hello Marc,
We observe the very similar issue with MCP2517FD.
>The CRC errors the patch works around are CRC errors introduced by a
>chip erratum, not by electromagnetic interference. In my observation
Are you referring this errata doc
https://datasheet.octopart.com/MCP2517FD-H-JHA-Microchip-datasheet-136609045.pdf ?
We have the similar CRC read errors but
the lowest byte is not 0x00 and 0x80, it's actually 0x0x or 0x8x, e.g.
mcp251xfd spi0.0 can0: CRC read error at address 0x0010 (length=4, data=82 d1 fa 6c, CRC=0xd9c2) retrying.
0xb0 0x10 0x04 0x82 0xd1 0xfa 0x6c => 0x59FD (not matching)
but if I flip the first received bit (highest bit in the lowest byte):
0xb0 0x10 0x04 0x02 0xd1 0xfa 0x6c => 0xD9C2 (matching!)
So, your fix covers only the case of 0x00 and 0x80,
do you think that the workaround should be extended so check
(buf_rx->data[0] == 0x0 || buf_rx->data[0] == 0x80)) {
turns into
((buf_rx->data[0] & 0xf0) == 0x0 || (buf_rx->data[0] & 0xf0) == 0x80)) {
Errata, actually says
"Only bits 7/15/23/31 of the following registers can be affected:"
So, we could basically, in simplest case flip bit 31 and re-check CRC without any check of
rx->data[0]....
Regards,
Pavel
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