On 11/11/19 12:09 AM, Drew Fustini wrote: > Hello, I'm looking at mcp251x driver and it does not seem to read the > device tree property > mcp251x,oscillator-frequency. > > I have a problem where I have MCP2515 with a 16MHz oscillator. However, > it is getting configured with a 8MHz clock despite > having mcp251x,oscillator-frequency defined as 16MHz: > https://github.com/pdp7/bb.org-overlays/blob/patch-1/src/arm/PB-MCP2515-SPI1.dts I assume you get the 8MHz clock rate from the "ip" output, right? > ip -details -statistics link show can0 > 8: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UP mode DEFAULT group default qlen 10 > link/can promiscuity 0 minmtu 0 maxmtu 0 > can state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0 > bitrate 500000 sample-point 0.875 > tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1 > usb_8dev: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..1024 brp-inc 1 > clock 32000000 ^^^^^^^^ Like here? (example output from a different CAN controller) > re-started bus-errors arbit-lost error-warn error-pass bus-off > 0 0 0 0 0 0 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 > RX: bytes packets errors dropped overrun mcast > 0 0 0 0 0 0 > TX: bytes packets errors dropped carrier collsns > 0 0 0 0 0 0 It's due to this line in the code: https://elixir.bootlin.com/linux/latest/source/drivers/net/can/spi/mcp251x.c#L1065 where only the half of the external osc frequency is stored. That value is displayed in the user space by the "ip" command. > I would appreciate any advice on whether a patch for mcp251x.c to > read mcp251x,oscillator-frequency would be a good way to solve this issue. Looking at the datasheet[1] of the mcp2515 page 40, equation 5-2, I think this is correct. As the timequanta is calculated by: Tq = (2 * Brp) / fosc On other words: Tq = Brp / (fosc / 2) We have no means of expressing the additional "/2" during the bit timing calculation, but to store the fosc/2 as the "effective" oscillator frequency. I successfully got a mcp25625 (which is basically a mcp2515 with internal phy) running on a rapsi using the frequency printed on the oscillator in the DT-overlay. [1] http://ww1.microchip.com/downloads/en/DeviceDoc/MCP2515-Stand-Alone-CAN-Controller-with-SPI-20001801J.pdf hth, 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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