Re: Linux board with 10 CANs

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 05/15/2015 10:20 AM, Yegor Yefremov wrote:
> On Fri, May 15, 2015 at 9:56 AM, François Beaulier <fbeaulier@orange.fr> wrote:
>> Hi,
>>
>> I'm starting the design of a CPU board, based on a computer on module with a
>> very common SOC like i.mx6 or am335x.
>> I need to fit my board with at least 10 CAN ports, may be more. (The SOC
>> have 2 CANs so i need 8 more)
>> The board will run with Linux and of course i want a socketcan interface for
>> each bus !
>> CAN bitrate needed is quite low (50kbps) and latency is not critical but bus
>> load may reach 100% sometimes.
>> As far as possible i want to avoid driver development, or just doing slight
>> modifications on an existing driver.
>> I don't want to use MCP2515, i had trouble with it on a previous design
>> because of the lack of buffer in the chip.
>> I can't imagine putting 8 MCP2515 and not missing any frame, but may be i'm
>> wrong ?
>>
>> Here are the options i have considered, they involve using external MCUs to
>> provide enough CAN interfaces :
>>
>> - MCUs with SLCAN : SLCAN have the advantage of simplicity and it should
>> offer enough performance at low data rate. Problem is that it would require
>> 8 UARTS. I could modify SLCAN driver to allow multiple CAN channels on the
>> same UART, and so use MCUs with multiple CANs, does it sounds reasonable ?
>>
>> - MCUs with SPI 1 : adapt MCP2515 driver in a way to make the SPI protocol
>> manageable on the MCU side (SPI slave). The MCU will be able to provide the
>> buffer for CAN frames that the MCP does not have. This option is just a way
>> of trying to go fast, it is not very elegant, and have the limitation of one
>> CAN channel per SPI connection / driver instance.
>>
>> - MCUs with SPI 2 : use the Stefano Babic SPI driver posted on this list.
>> This is probably the best choice for a long term and more generic solution.
>> But i'm afraid it may require too much work to complete. What is the status
>> of this driver right now ? Does anyone have an idea of the remaining work to
>> do to make it ready for mainline ? Stefano are you available to spend more
>> time on it ? I'm also wondering whether multiple instances of the driver can
>> run concurently if i put more than one MCU ?
>>
>> Concerning the MCU software i could release it as an open-source project,
>> anyone interested in participating ?
>> I'm very familiar with STM32 MCUs so i will probably use an STM32F072 (low
>> cost cortex M0 with one CAN).
>>
>> Any idea or advice is welcome, thank you for reading this much too long
>> post.
> 
> Is it possible to attach 8 SJA1000s over local bus? The driver is
> already in mainline kernel.

Don't use the mcp251x.

If you have PCIe, there are some dual CAN modules (available in miniPIC,
too). But PCIe ist probably quite cost intensive. USB would be an
option, but I'm not sure if there are modules that can be
connected/soldered to a PCB. As Yegor said, SJA1000s + a CPLD are an
option, too, but IIRC the SJA1000 has a 5V IO interface.

Marc

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