[J-core] Checking in.

Tue Dec 17 05:55:55 UTC 2019

Sorry, gmail's spam filter ate this for some reason. Just fished it out.

On 12/6/19 2:26 PM, wjones at wdj-consulting.com wrote:
>> In the existing bitstream? One. The USB-A charge connection thingy has a serial
>> port on it, which the kernel serial console defaults to. (I.E. plug it into a
>> laptop or powered hub and it's both the power source and a serial data connection.)
>
> In my setup, it's _probably_ not going to be feasible to use a serial port
> that's shared with the power connection. I plan to plug the TurtleBoard directly
> into an outlet. What is the current draw of the TurtleBoard under load if you
> know? I can _try_ it and see what happens, of course.

Um, I don't have a tool that shows me the current draw on a USB port. Is there a
way to get Linux to tell you that? (Although the USB stack probably only knows
what it's negotiated, not how much it's actually _using_...)

Martin or Jeff might have real figures?

>> Here's an old post about modifying the bitstream to put serial ports on the GPIO
> pins:
> http://lists.j-core.org/pipermail/j-core/2016-April/000070.html
> 
> This is very good to know, I'm probably going to have to go this route and
> redirect the kernel console to this port.

The kernel console info is on the kernel command line. I believe we're supplying
the kernel command line via device tree, thus it's in the .dtb file on the sd
card? (That's a Rich Felker question, really.)

The ultimate source of it is CONFIG_CMDLINE="console=ttyUL0 earlycon"
in arch/sh/configs/j2_defconnfig in the kernel source, so presumably if you
change that and rebuild everything and replace all the files, it should just
work. But I _think_ the .dtb file is the important one for this...)

>> We have a TODO item to hook up DMA to the serial port in the kernel, but haven't
> done it yet.
> Not a big deal, even on my most powerful SBC (Tinker Board), DMA on the serial
> port doesn't work :P. I'll manage without it.

If you're using the serial port as your network stack, you're gonna want a big
FIFO and DMA if at all possible. (And to dial the speed up as fast as possible.
I dunno where the clock we're using for serial currently maxes out, Jeff and I
talked about this last year but I have no idea where the notes on it would be.
In _theory_ I think we can do 4 megabit, in practice I dunno what kind of
pentagrams and dribbly candles are required to make that work. That's a jeff
question.)

>> I could add a getty to toybox that does it, I just didn't know there was demand?
>
> I would appreciate that; I know my use case is niche, but PPP has actually saved
> me a number of times where the a serial line (combined with proxy ARP) is the
> only convenient way to get Internet access. A multiplexing getty, _once it's set
> up_ :), makes debugging issues _a lot_ easier.

Um, what's involved in setting it up?

I've added it to my todo list (actually a comment in toys/pending/gettty.c) but
I haven't used this before so have to research what success looks like. I'm
guessing from https://tools.ietf.org/html/rfc1661#section-3.4 the handoff is
basically recognizing a link configure packet and calling the ppp daemon, but
the last time I set up ppp was actually pppoe and it was last decade.)

Rob

> 
> -----Original Message----- From: Rob Landley
> Sent: Friday, December 06, 2019 3:09 PM
> To: wjones at wdj-consulting.com
> Cc: j-core at lists.j-core.org
> Subject: Re: [J-core] Checking in.
> 
> On 12/6/19 6:48 AM, wjones at wdj-consulting.com wrote:
>> While this is on my mind...
>>
>> How many UARTs are available on TurtleBoard?
> 
> In the existing bitstream? One. The USB-A charge connection thingy has a serial
> port on it, which the kernel serial console defaults to. (I.E. plug it into a
> laptop or powered hub and it's both the power source and a serial data connection.)
> 
> It's got 4 standard USB ports, each of which can have a serial adapter plugged
> into it, or add a USB hub and I think you can have 127 devices?
> 
> Here's an old post about modifying the bitstream to put serial ports on the GPIO
> pins:
> 
>  http://lists.j-core.org/pipermail/j-core/2016-April/000070.html
> 
> You might need an i2c to uart converter chip depending on what electrical
> signals you want to hook it to at the other end, but Jeff didn't use one when he
> slapped one of those together on a numato board that had a bad serial connector
> so he rebuilt the bitstream for a demo an hour beforehand. (He just stuck wires
> on the pins and it worked for him? You need the stty command to disable
> hardaware flow control, but you needed that on numato anyway. Unless you wire up
> the 4 pin version in the bitstream...)
> 
>> I don't actually plan to use the
>> Ethernet on my TurtleBoard. Instead, I plan use PPP to connect it to the
>> Internet (it'll connect a UART to a wifi-enabled board's secondary UART). If
>> there is only room for one UART plus Ethernet, is it possible to accommodate my
>> requirement and replace Ethernet with a second UART in the bitstream?
> 
> UARTs are tiny and even the _small_ turtle is LX25: that's almost 3 times the
> size of the LX9 in the numato board. And the LX45 version is 5 times the size of
> the LX9.
> 
> We have a TODO item to hook up DMA to the serial port in the kernel, but haven't
> done it yet.
> 
>> The (Net)BSD getty is smart enough to multiplex pppd and login when only a
>> single UART is available, but I don't believe any Linux getty implementations
>> have this feature...
> 
> I could add a getty to toybox that does it, I just didn't know there was demand?
> 
> Jeff flew to Japan yesterday and I haven't heard from him since (probably
> collapsed as soon as he got there, 10 hour time difference from Ontario and he
> flew out after noon), but he's the guy to ask about the ship schedule. The
> hardware's here, the blocker is us fleshing out the bitstream and doing the
> packaging and documentation and such...
> 
> Rob
> 
> -- 
> William D. Jones
> wjones at wdj-consulting.com
> 