netdev - Re: [PATCH v2] can: ctucanfd: fix SSP_SRC in cases when bit-rate is higher than 1 MBit.

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Message-ID: <c5851986-837b-4ffb-9bf7-3131cf9c05d1@kernel.org>
Date: Mon, 5 Jan 2026 21:27:11 +0100
From: Vincent Mailhol <mailhol@...nel.org>
To: Pavel Pisa <pisa@....cvut.cz>, linux-can@...r.kernel.org,
 Marc Kleine-Budde <mkl@...gutronix.de>,
 David Laight <david.laight.linux@...il.com>,
 "David S. Miller" <davem@...emloft.net>,
 Andrea Daoud <andreadaoud6@...il.com>,
 Wolfgang Grandegger <wg@...ndegger.com>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org
Cc: Jiri Novak <jnovak@....cvut.cz>, Ondrej Ille <ondrej.ille@...il.com>
Subject: Re: [PATCH v2] can: ctucanfd: fix SSP_SRC in cases when bit-rate is
 higher than 1 MBit.

Le 05/01/2026 à 12:16, Pavel Pisa a écrit :
> From: Ondrej Ille <ondrej.ille@...il.com>
> 
> The Secondary Sample Point Source field has been
> set to an incorrect value by some mistake in the
> past
> 
>   0b01 - SSP_SRC_NO_SSP - SSP is not used.
> 
> for data bitrates above 1 MBit/s. The correct/default
> value already used for lower bitrates is

Where does this 1 MBit/s threshold come from? Is this an empirical value?

The check is normally done on the data BRP. For example we had some
problems on the mcp251xfd, c.f. commit 5e1663810e11 ("can: mcp251xfd:
fix TDC setting for low data bit rates").

Can you use the TDC framework? Not only would you get a correct
calculation for when to activate/deactivate TDC, you will also have the
netlink reporting (refer to the above commit for an example).

>   0b00 - SSP_SRC_MEAS_N_OFFSET - SSP position = TRV_DELAY
>          (Measured Transmitter delay) + SSP_OFFSET.
> 
> The related configuration register structure is described
> in section 3.1.46 SSP_CFG of the CTU CAN FD
> IP CORE Datasheet.
> 
> The analysis leading to the proper configuration
> is described in section 2.8.3 Secondary sampling point
> of the datasheet.
> 
> The change has been tested on AMD/Xilinx Zynq
> with the next CTU CN FD IP core versions:
> 
>  - 2.6 aka master in the "integration with Zynq-7000 system" test
>    6.12.43-rt12+ #1 SMP PREEMPT_RT kernel with CTU CAN FD git
>    driver (change already included in the driver repo)
>  - older 2.5 snapshot with mainline kernels with this patch
>    applied locally in the multiple CAN latency tester nightly runs
>    6.18.0-rc4-rt3-dut #1 SMP PREEMPT_RT
>    6.19.0-rc3-dut
> 
> The logs, the datasheet and sources are available at
> 
>  https://canbus.pages.fel.cvut.cz/
> 
> Signed-off-by: Ondrej Ille <ondrej.ille@...il.com>
> Signed-off-by: Pavel Pisa <pisa@....cvut.cz>

Yours sincerely,
Vincent Mailhol