Re: Deciphering GPIO on Raspberry Pi B+

[Jason Thorpe <thorpej%me.com@localhost>]

> On Apr 15, 2018, at 1:34 AM, Hal Murray <hmurray%megapathdsl.net@localhost> wrote:
> 
> Are you trying to solve a one-off problem so you can go back to working on 
> your problem or the general problem so the next guy doesn't have to work so 
> hard?

I’d prefer the latter, of course. :-)

> I think the right answer to the general problem is that the pre-boot software 
> that sets up the IO devices should provide a bit-mask of bits that are 
> available for use as GPIO with the control block for that GPIO device.  I 
> don't know if it does that yet.

I would assume this is available somewhere in the FDT.

> You maybe able to write a user-land version of the dtb code that would do 
> everything but actually configure the hardware and add the leftover pins 
> feature to it.  That assumes you can figure out what was input to the real 
> dtb code back at boot time.

I’d be happy with just a way to query this using the GPIO ioctl API.  It’s kind of annoying that one can’t even query it for any meaningful info at kern.securelevel > 0.

> It might be possible to write some code to try them all and see which ones 
> work.  The idea is to write 0s and 1s and see if they read back correctly.  
> The builtin pullups may work too well.  I don't know if there is a way to 
> turn them off and back on.

I already discovered that e.g. GPIO6 doesn’t seem to work, even though it’s brought out to the header and has a non-interesting set of alternate functions.  I settled on GPIO23, GPIO24, and GPIO25 for now (to drive a chain of 74HC595s).

> 
> 
> mlelstv%serpens.de@localhost said:
>> -> https://elinux.org/RPi_BCM2835_GPIOs 
> 
> That has a pointer to the hardware documentation.
>  http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripheral
> s.pdf
> 
> ---------
> 
> I have an Adafruit GPS board handy.  It's got a prototyping area, and labels 
> next to a bunch of holes next to the connector.  Some of the labels are 
> things like SDA, SCL, TXD, and RXD.  Some are things like #4, #17, #18, #27, 
> #22, #23, #24, #25 #5, #6, #12, #13, #16, #19, #20, and #21. I assume those 
> are GPIO numbers, but I can't make a match with that list and the URL above.

Which Adafuit GPS board, specifically?  (I think they make a couple of different ones…)

-- thorpej