netdev - RE: [PATCH net v1] net: wan: fsl_qmc

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Message-ID: <7659191688194f099cba22cc367c4eda@AcuMS.aculab.com>
Date: Fri, 2 Aug 2024 08:10:02 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Simon Horman' <horms@...nel.org>, Herve Codina <herve.codina@...tlin.com>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Christophe Leroy <christophe.leroy@...roup.eu>, "Andy
 Shevchenko" <andriy.shevchenko@...ux.intel.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Thomas Petazzoni
	<thomas.petazzoni@...tlin.com>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>
Subject: RE: [PATCH net v1] net: wan: fsl_qmc_hdlc: Discard received CRC

From: Simon Horman
> Sent: 31 July 2024 09:45
> 
> On Tue, Jul 30, 2024 at 08:31:33AM +0200, Herve Codina wrote:
> > Received frame from QMC contains the CRC.
> > Upper layers don't need this CRC and tcpdump mentioned trailing junk
> > data due to this CRC presence.
> >
> > As some other HDLC driver, simply discard this CRC.
> 
> It might be nice to specifically site an example.
> But yes, I see this pattern in hdlc_rx_done().

Pretty much the only reason you'd want the received CRC is to do
error recovery assuming a single short error burst.
Not that I've ever seen a driver contain the required code.
'Back of envelope' calculation: 32bit crc, 2kbyte frame, so 18bits
of crc per data bit, sqrt for 'birthday paradox' - I bet you can
recover 8-bit error bursts.

	David

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