On Wednesday 18 October 2017 07:47 PM, Franklin S Cooper Jr wrote:

On 10/18/2017 08:24 AM, Sekhar Nori wrote:

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Hi Marc,

On Wednesday 18 October 2017 06:14 PM, Marc Kleine-Budde wrote:

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On 09/21/2017 02:48 AM, Franklin S Cooper Jr wrote:

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On 09/20/2017 04:37 PM, Mario Hüttel wrote:

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On 09/20/2017 10:19 PM, Franklin S Cooper Jr wrote:

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Hi Wenyou,

On 09/17/2017 10:47 PM, Yang, Wenyou wrote:

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On 2017/9/14 13:06, Sekhar Nori wrote:

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On Thursday 14 September 2017 03:28 AM, Franklin S Cooper Jr wrote:

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On 08/18/2017 02:39 PM, Franklin S Cooper Jr wrote:

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During test transmitting using CAN-FD at high bitrates (4 Mbps) only
resulted in errors. Scoping the signals I noticed that only a single
bit
was being transmitted and with a bit more investigation realized the
actual
MCAN IP would go back to initialization mode automatically.

It appears this issue is due to the MCAN needing to use the Transmitter
Delay Compensation Mode as defined in the MCAN User's Guide. When this
mode is used the User's Guide indicates that the Transmitter Delay
Compensation Offset register should be set. The document mentions
that this
register should be set to (1/dbitrate)/2*(Func Clk Freq).

Additional CAN-CIA's "Bit Time Requirements for CAN FD" document
indicates
that this TDC mode is only needed for data bit rates above 2.5 Mbps.
Therefore, only enable this mode and only set TDCO when the data bit
rate
is above 2.5 Mbps.

Signed-off-by: Franklin S Cooper Jr <redacted>
---
I'm pretty surprised that this hasn't been implemented already since
the primary purpose of CAN-FD is to go beyond 1 Mbps and the MCAN IP
supports up to 10 Mbps.

So it will be nice to get comments from users of this driver to
understand
if they have been able to use CAN-FD beyond 2.5 Mbps without this
patch.
If they haven't what did they do to get around it if they needed higher
speeds.

Meanwhile I plan on testing this using a more "realistic" CAN bus to
insure
everything still works at 5 Mbps which is the max speed of my CAN
transceiver.

ping. Anyone has any thoughts on this?

I added Dong who authored the m_can driver and Wenyou who added the only
in-kernel user of the driver for any help.

I tested it on SAMA5D2 Xplained board both with and without this patch, 
both work with the 4M bps data bit rate.

Thank you for testing this out. Its interesting that you have been able
to use higher speeds without this patch. What is the CAN transceiver
being used on the SAMA5D2 Xplained board? I tried looking at the
schematic but it seems the CAN signals are used on an extension board
which I can't find the schematic for. Also do you mind sharing your test
setup? Were you doing a short point to point test?

Thank You,
Franklin

Hello Franklin,

your patch definitely makes sense.

I forgot the TDC in my patches because it was not present in the
previous driver versions and because I didn't encounter any
problems when testing it myself.

The error is highly dependent on the hardware (transceiver) setup.
So it is definitely possible that some people don't encounter errors
without your patch.

So the Transmission Delay Compensation feature Value register is suppose
to take into consideration the transceiver delay automatically and add
the value of TDCO on top of that. So why would TDCO be dependent on the
transceiver? I've heard conflicting things regarding TDC so any
clarification on what actually impacts it would be appreciated.

Also part of the issue I'm having is how can we properly configure TDCO?
Configuring TDCO is essentially figuring out what Secondary Sample Point
to use. However, it is unclear what value to set SSP to and which use
cases a given SSP will work or doesn't work. I've seen various
recommendations from Bosch on choosing SSP but ultimately it seems they
suggestion "real world testing" to come up with a proper value. Not
setting TDCO causes problems for my device and improperly setting TDCO
causes problems for my device. So its likely any value I use could end
up breaking something for someone else.

Currently I leaning to a DT property that can be used for setting SSP.
Perhaps use a generic default value and allow individuals to override it
via DT?

Sounds reasonable. What's the status of this series?

I have had some offline discussions with Franklin on this, and I am not
fully convinced that DT is the way to go here (although I don't have the
agreement with Franklin there).

Probably the fundamental area where we disagree is what "default" SSP
value should be used. Based on a short (< 1 ft) point to point test
using a SSP of 50% worked fine. However, I'm not convinced that this
default value of 50% will work in a more "traditional" CAN bus at higher
speeds. Nor am I convinced that a SSP of 50% will work on every MCAN
board in even the simplest test cases.

I believe that this default SSP should be a DT property that allows any
board to determine what default value works best in general.

With that, I think, we are taking DT from describing board/hardware
characteristics to providing default values that software should use.

In any case, if Marc and/or Wolfgang are okay with it, binding
documentation for such a property should be sent to DT maintainers for
review.

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There are two components in configuring the secondary sample point. It
is the transceiver loopback delay and an offset (example half of the bit
time in data phase).

While the transceiver loopback delay is pretty board dependent (and thus
amenable to DT encoding), I am not quite sure the offset can be
configured in DT because its not really board dependent.

Unfortunately, offset calculation does not seem to be an exact science.
There are recommendations ranging from using 50% of bit time to making
it same as the sample point configured. This means users who need to
change the SSP due to offset variations need to change  their DT even
without anything changing on their board.

Since we have a netlink socket interface to configure sample point, I
wonder if that should be extended to configure SSP too (or at least the
offset part of SSP)?

Sekhar is right that ideally the user should be able to set the SSP at
runtime. However, my issue is that based on my experience CAN users
expect the driver to just work the majority of times. For unique use
cases where the driver calculated values don't work then the user should
be able to override it. This should only be done for a very small
percentage of CAN users. Unless you allow DT to provide a default SSP
many users of MCAN may find that the default SSP doesn't work and must
always use runtime overrides to get anything to work. I don't think that
is a good user experience which is why I don't like the idea.

Fair enough. But not quite sure if CAN users expect CAN-FD to "just
work" without doing any bittiming related setup.

Thanks,
Sekhar