netdev - Re: [PATCH v5 2/7] can: bittiming: allow TDC{V,O} to be zero and add can_tdc_const::tdc{v,o,f}

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Date:   Mon, 16 Aug 2021 14:33:09 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     linux-can <linux-can@...r.kernel.org>,
        Stefan Mätje <Stefan.Maetje@....eu>,
        netdev <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/7] can: bittiming: allow TDC{V,O} to be zero and add
 can_tdc_const::tdc{v,o,f}_min

On 16.08.2021 14:25:19, Marc Kleine-Budde wrote:
> > > I'm not sure, if we talked about the mcp251xfd's tcdo, valid values are
> > > -64...63.
> > 
> > Yes! Stefan shed some light on this. The mcp251xfd uses a tdco
> > value which is relative to the sample point.
> 
> I don't read the documentation this way....
> 
> > | SSP = TDCV + absolute TDCO
> > |     = TDCV + SP + relative TDCO
> > 
> > Consequently:
> > | relative TDCO = absolute TDCO - SP
> 
> In the mcp15xxfd family manual
> (http://ww1.microchip.com/downloads/en/DeviceDoc/MCP251XXFD-CAN-FD-Controller-Module-Family-Reference-Manual-20005678B.pdf)
> in the 2mbit/s data bit rate example in table 3-5 (page 21) it says:
> 
> | DTSEG1  15 DTQ
> | DTSEG2   4 DTQ
> | TDCO    15 DTQ
> 
> The mcp251xfd driver uses 15, the framework calculates 16 (== Sync Seg+
> tseg1, which is correct), and relative tdco would be 0:
> 
> | mcp251xfd_set_bittiming: tdco=15, priv->tdc.tdc=16, relative_tdco=0
> 
> Here the output with the patched ip tool:

Sorry, the previous output was not using the sample points of the
example in the data sheet, this is the fixed output:

| 6: mcp251xfd0: <NOARP,UP,LOWER_UP,ECHO> mtu 72 qdisc pfifo_fast state UP mode DEFAULT group default qlen 10
|     link/can  promiscuity 0 minmtu 0 maxmtu 0 
|     can <FD,TDC_AUTO> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 100 
|           bitrate 500000 sample-point 0.800
|           tq 25 prop-seg 31 phase-seg1 32 phase-seg2 16 sjw 1 brp 1
|           mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp_inc 1
|           dbitrate 2000000 dsample-point 0.800
|           dtq 25 dprop-seg 7 dphase-seg1 8 dphase-seg2 4 dsjw 1 dbrp 1
|           tdco 16
|           mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbrp_inc 1
|           tdco 0..127
|           clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 parentbus spi parentdev spi0.0 

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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