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XPost: sci.electronics.design
On 21/09/2024 20:56, john larkin wrote:
> On Sat, 21 Sep 2024 19:50:34 -0000 (UTC), Phil Hobbs
> wrote:
>
>> john larkin wrote:
>>> On Sat, 21 Sep 2024 19:29:26 +0100, The Natural Philosopher
>>> wrote:
>>>
>>>> On 21/09/2024 16:08, john larkin wrote:
>>>>> On Sat, 21 Sep 2024 09:12:06 +0100, The Natural Philosopher
>>>>> wrote:
>>>>>
>>>>>> On 20/09/2024 19:00, john larkin wrote:
>>>>>>> On 20 Sep 2024 11:30:13 +0100 (BST), Theo
>>>>>>> wrote:
>>>>>>>
>>>>>>>> In comp.sys.raspberry-pi The Natural Philosopher wrote:
>>>>>>>>> On 19/09/2024 23:09, Lasse Langwadt wrote:
>>>>>>>>>> On 9/18/24 00:33, john larkin wrote:
>>>>>>>>>
>>>>>>>>>>> It looks like a USB memory stick. You can delete or add files if
you
>>>>>>>>>>> want.
>>>>>>>>>>>
>>>>>>>>>>> It boots CPU 0 (the one we call Alice) from a file with the
extension
>>>>>>>>>>> .UL2
>>>>>>>>>>>
>>>>>>>>>>> Why .UL2 one wonders.
>>>>>>>>>>>
>>>>>>>>>>> We'll put a bunch of files into the flash. Code for Bob, the 2nd
CPU.
>>>>>>>>>>> An FPGA bitstream file. A prototype calibration table. A README
file
>>>>>>>>>>> to explain everything in plain English.
>>>>>>>>>>
>>>>>>>>>> sure it's not UF2?
>>>>>>>>>>
>>>>>>>>>> https://github.com/microsoft/uf2
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>> Definitely uf2 here.
>>>>>>>>>
>>>>>>>>> And no, you cannot 'delete or add files' to it.
>>>>>>>>> The action of pretending to download a uf2 file into what appears to
be
>>>>>>>>> an empty drive, erases everything on it and programs the flash.
>>>>>>>>>
>>>>>>>>> There are no visible files to delete.
>>>>>>>>
>>>>>>>> Neat. So basically you throw some files at it, which causes a series
of
>>>>>>>> block writes. UF2 picks out specially tagged block writes and uses
that to
>>>>>>>> program the flash. It doesn't actually care what other stuff is
written to
>>>>>>>> the flash as it ignores all of that, so it doesn't care about all the
FAT
>>>>>>>> stuff or whatever junk your OS decides to put on there.
>>>>>>>>
>>>>>>>> Means you can write any kind of files to it and it'll only pay
attention to
>>>>>>>> the specific tagged blocks. If the OS is happy to cache the medium
(as many
>>>>>>>> do) you could maybe even reformat it as some other filesystem like
NTFS and
>>>>>>>> it would still handle writing the UF2 file correctly.
>>>>>>>>
>>>>>>>> Theo
>>>>>>>
>>>>>>> My Pi guy says that you can only write one file, and the act of
>>>>>>> writing that file wipes anything that was there before. So the flash
>>>>>>> probably doesn't have a file structure, and the USB memory-stick write
>>>>>>> is, well, a sort of cheap trick.
>>>>>>>
>>>>>>> That's workable, if inelegant. We can pack everything we need into
>>>>>>> that one big file and users can upgrade box code in the field pretty
>>>>>>> easily.
>>>>>>>
>>>>>>
>>>>>> It gets nastier if you want to preserve config info across reboots.
>>>>>> It is possible to read and write areas of flash from the code, but its
>>>>>> no picnic.
>>>>>> And it gets wiped when new code is uploaded
>>>>>>
>>>>>>
>>>>>> It is an area I will have to tackle for one project tho.
>>>>>
>>>>> Yes, writing to flash from the running application is nasty.
>>>>>
>>>>> We have to calibrate each box. We'll store the prototype calibration
>>>>> table inside the big flash image. At factory test, we'll grab that,
>>>>> edit it for this particular unit, and save it to a small SPI eeprom
>>>>> chip. That costs 24 cents and one chip select pin.
>>>>>
>>>>> My guy says that there are a few magic integers at the start of the
>>>>> UF2 file that identifies it, well, as a UF2 file. That confirms that
>>>>> the Pico flash doesn't have a file structure, it just stores one giant
>>>>> chunk of stuff starting at the start.
>>>>>
>>>>> It's Windows who lies about it acting like a USB memory stick that
>>>>> stores files.
>>>>>
>>>>> We did consider saving the real cal table at some fixed physical
>>>>> address near the end of the flash , on the theory that nobody will
>>>>> ever write a bootable image that big. That might work.
>>>>>
>>>> That seems to be the case.
>>>>
>>>> I looked into it enough to see that it would be possible to store NV
>>>> data in a high part of the flash.
>>>>
>>>> I think that the runtime provides access to a memory location that
>>>> indicates the end of the uploaded flash image, so in theory flash above
>>>> that is free to write, with the proviso it has to be done in large
>>>> blocks on specific address boundaries.
>>>>
>>>> All this is at least Pi Pico specific anyway.
>>>
>>> We're using the RP2040 chip, so will have a huge flash chip. We will
>>> sometimes store an FPGA config file that could be too big for the 2
>>> MByte part on the Pico.
>>>
>>>
>>>>
>>>> Will keep me busy through the dark winter days...:-)
>>>
>>> Storing anything in high flash still has the problem that you can't
>>> run flash-cached code while the write is going on, unless you are very
>>> careful.
>>>
>>>
>>
>> It’s good to have a warm relationship with your linker mapfile. ;)
>>
>> Cheers
>>
>> Phil Hobbs
>
> Interrupts might get nasty, demanding swaps into the flash cache when
> the flash is busy writing.
>
I think the recommended technique is to disable all those and suspend
any other threads that might be active or write only single threaded code
--
“But what a weak barrier is truth when it stands in the way of an
hypothesis!”
Mary Wollstonecraft
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