From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wolfgang Grandegger Subject: Re: About bittiming calculation result Date: Mon, 07 Feb 2011 16:52:53 +0100 Message-ID: <4D501555.5000905@grandegger.com> References: <20110204124233.GB334@e-circ.dyndns.org> <20110204.130649.112613896.davem@davemloft.net> <5009516791F146C49C73FAC57C437313@hacdom.okisemi.com> <4D4FDEF9.2030305@grandegger.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: socketcan-core-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org, netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Tomoya MORINAGA Return-path: In-Reply-To: <4D4FDEF9.2030305-5Yr1BZd7O62+XT7JhA+gdA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: socketcan-core-bounces-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org Errors-To: socketcan-core-bounces-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org List-Id: netdev.vger.kernel.org On 02/07/2011 01:00 PM, Wolfgang Grandegger wrote: > Hi Tomoya, > > On 02/07/2011 12:38 PM, Tomoya MORINAGA wrote: >> Hi, >> >> I have a question for bittiming-value calculated by Can-core. >> >> In case setting like below, >> - ip link set can0 type can bitrate 800000 >> - clock=50MHz >> - Use pch_can >> >> Can-core calculates like below >> brp=21 >> seg1=1 >> seg2=1 >> sjw=1 >> prop_seg=0 >> >> Is "prop_seg=0" true ? > > Well, only prop_seg+phase_seg=tseg1 is relevant and the pch_can driver > sets the allowed minimum "tseg1_min1" currently to 1: > > static struct can_bittiming_const pch_can_bittiming_const = { > .name = KBUILD_MODNAME, > .tseg1_min = 1, > .tseg1_max = 16, > .tseg2_min = 1, > .tseg2_max = 8, > .sjw_max = 4, > .brp_min = 1, > .brp_max = 1024, /* 6bit + extended 4bit */ > .brp_inc = 1, > }; > >> seg1/seg2/sjw/prop_seg must be more than 1 ? > > Then "tseg1_min" should be set to *2*. > >> Also I can see the following kernel error log. >> bitrate error 0.7% > > A clock frequency of 50 MHz is sub-optimal for CAN and some > bit-rates cannot be reproduced properly. Here is the output of > the can-utils program "can-calc-bit-timing" (with an entry for > the pch-can added): > > $ ./can-calc-bit-timing pch-can > Bit timing parameters for pch-can with 50.000000 MHz ref clock > nominal real Bitrt nom real SampP > Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error CNF1 CNF2 CNF3 > 1000000 100 3 3 3 1 5 1000000 0.0% 75.0% 70.0% 6.7% 0x05 0x92 0x02 > 800000 420 0 1 1 1 21 793650 0.8% 80.0% 66.6% 16.8% 0x15 0xff 0x00 > 500000 100 8 8 3 1 5 500000 0.0% 87.5% 85.0% 2.9% 0x05 0xbf 0x02 > 250000 500 3 3 1 1 25 250000 0.0% 87.5% 87.5% 0.0% 0x19 0x92 0x00 > 125000 500 6 7 2 1 25 125000 0.0% 87.5% 87.5% 0.0% 0x19 0xb5 0x01 > 100000 500 8 8 3 1 25 100000 0.0% 87.5% 85.0% 2.9% 0x19 0xbf 0x02 > 50000 2500 3 3 1 1 125 50000 0.0% 87.5% 87.5% 0.0% 0x7d 0x92 0x00 > 20000 2500 8 8 3 1 125 20000 0.0% 87.5% 85.0% 2.9% 0x7d 0xbf 0x02 > 10000 12500 3 3 1 1 625 10000 0.0% 87.5% 87.5% 0.0% 0x71 0x92 0x00 > > As you can see, especially 800000 gives rather bad results. BTW, it's always possible to specify optimized bit-timing parameters directly, e.g. the following seem better: 800000 60 12 4 4 4 3 793650 0.8% 80.0% 81.0% 1.2% You could set these with: $ ip link set can0 type can \ tq 60 prop-seg 12 phase-seg1 4 phase-seg2 4 sjw 4 Wolfgang.