hi aaron, thanks for joining in here. most welcome.
On Sun, Nov 3, 2013 at 9:34 AM, Aaron J. Seigo aseigo@kde.org wrote:
Hi...
On the eoma68 spec it says:
"The EEPROM MUST NOT be used for the storage of user data: it is reserved exclusively for EOMA-68."
yes. my concerns here are that the EEPROM would be overwritten by user applications otherwise, potentially destroying the ability of CPU Cards to identify the device. as it is i'm concerned about the possibility of android viruses resulting in the id info being destroyed and ending up with mass product recalls. if it wasn't for the possibility of kernel upgrades breaking device-tree file format or interoperability i would have said it has to be a one-time fusable PROM [or at least there being a factory-install write protect on the EEPROM].
the hardware's not going to change, so why would the data representing it change?
My understanding from past threads on the matter is that currently the only specified usage for this by EOMA-68 is a product ID string + device node tree.
there was some debate about whether device node trees *plural* would be more appropriate - one per "thing". e.g. "GPIO pin 5" would have its own device-tree node describing what it was for. "LCD" would have its own device-node tree in lieu of most LCDs not properly supporting EDID [and even when they do they often have totally false data] and so on.
the other possibility being considered was to turn that on its head and define "purposes". such as a "Reset" purpose, to which "GPIO5" happens to be allocated. and "display", to which "LCD" happens to be allocated. this would make more sense inasmuch as some hardware devices will have e.g. DisplayLink USB-based screens, but then those are USB and they're self-describing so are out-of-scope _anyway_.
It would be most useful if there was a definition on the layout.
indeed! it's been very much a "we'll cross that bridge when we get to it" thing - and that's almost time, now.
*cough*
:)
A defined data structure allowing for future expansion
yes. this can be something as simple as a bit field (i learned a trick or two from microsoft and the DCE/RPC specification). the trick is describe "capabilities" rather than have "revision numbers" - how can future hardware / software otherwise support interoperability? i've been working with Ettus Research USRPs recently and their hard-coded use of "revision numbers" for communications formats between firmware and the libraries is a downgrade-upgrade nightmare. so. yes. bit-field with "capabilities" where the last bit is reserved for "yet more future expansion" a la escape-sequence.
as well as vendor specific additions would be golden.
ah. right. knowing partly the answer already [and thank you for agreeing to join the list to discuss this!], i have to ask, for the benefit of the list: what _kind_ of vendor-specific additions? specific examples, general examples, doesn't matter.
bear in mind that the EEPROM is only 32k in size, it's a $0.10 part, and its main purpose is to store this critical missing identification data that all the other EOMA68 buses don't need because they're self-describing.
A simple page based system would suffice afaics. Sth like:
Address Size Value ====== === ==== 0x00 2 Page identifier (defined by EOMA68 spec) 0x02 2 Size of page data 0x06 val 0x02 Page data
Page data would (obviously) be limited to a maximum 64k in size, but that really should not be an issue given the application (and given how much data you can read at a time via i2c_smbus calls). A checksum could be added to the page header if desired to verify the data read in, but that could also be left to the page specific data. I don’t think a checksum is needed for the device ID page, for instance.
i recommend having at least one byte saying "capabilities", as the first byte. then you can read that first byte and go "oh. bit {say} 5 is set! that means we use the new-spangly-expanded-data-format" which happens to be that you use 32-bit addressing not 16-bit for example. 8th bit you set to be "escape sequence".
then, yes! you can set one of the bits as "there is a checksum". another way to do this is to have a checksum "page". hmm, so many possibilities...
Page identifiers would be things like: "0x01 Device ID".
originally i was going to follow the principle of USB class : vendor. but now that i think about it this is best reserved for an actual in-page piece of information. so there would be "0x01 Device ID Page Info" then a page with that type would have... oh you describe it below already :)
A simple, sequential page ordering would to make it easy to use a function lookup table to parse the data. Page 0x00 would be a special “there are no more pages” id.
... giving a simple linked list in effect. yes. like it.
The page ID # space could be split in 2 between EOMA68 and vendor, giving 32768 possible page IDs to both (far more than necessary, but 1 byte isn’t enough, and i don’t see any benefit to trying to pack bytes together and bit twiddling on the software side to unpack them).
true.
this would have the nice characteristic of any page id with the high bit set to 1 would be a vendor page.
... but, again, before getting to that, i'd like to determine what those vendor-specific applications are, and to discuss whether it's appropriate to even have them. given the potential risks of having the hardware destroyed by user applications getting it wrong i'm inclined very much towards the "absolutely not" end of the spectrum.
i owned a Motorola A1200. actually i owned two. one of them was already a brick (ebay purchase... *sigh*...) in the end i worked out what happened, after i had the phone switched off over a timezone change. basically when i powered the phone up after a prolonged period of disuse, it tried to overwrite its own firmware, got it wrong, and destroyed itself. that was _two_ $300 costly bricks i owned. both of them out of warranty.
so for safety, even _allowing_ vendors to write to the EEPROM is something that i would like to see *actively* prevented.
if vendors *really* want some non-volatile space, there's a couple of options. 1) some NAND/NOR 2) a second EEPROM [with a different address] 3) other [e.g. a proper crypto-chip].
these I2C EEPROMs are only $0.10 to $0.15 so it's not a huge deal to put a 2nd one down on any board.
The device ID page data definition might look sth like:
Size in Byte(s) Value ========== ====================== 2 Device type (assigned centrally) 2 Device revision 4 Vendor ID (assigned centrally) 4 Product ID (vendor generated, registered centrally) N Manufacturer cstring N Product cstring N Serial number cstring
can i think of anything else here.... no i think you have it covered. the guiding principle here is the USB spec / consortium. and the linux kernel MACHINE_TYPE registry.
A set of defined pages would be marked as REQUIRED and standard page IDs would be defined as well. I’d actually recommend this definition to be housed *outside* of the EOMA68 spec so that it can be used more broadly by the open hardware community (where such things seem non-existent) and to keep the EOMA68 spec from becoming too monstrous.
yes. 2nd spec. and separate registry.
This approach would have a number of advantages imo:
- quick access to any given page without lots of index overhead (just jump
through the page headers)
yep.
- minimum size is self documenting: all REQUIRED pages (e.g. device ID page)
become the minimum size for the EEPROM on the device
yep.
- It avoids (imho) stupid hard coded lengths;
duh.
it’s no longer 1985, even in the embedded world people and many/most of the products we’re working on have application class processors, and even the ones with “just” microcontrolers can do a hell of a lot such that hard coded page sizes just don’t make sense
- It allows some vendor flexibility
... let's discuss this separately ok?
I am willing to write a small C library that takes a well formed set of data, parses all the EOMA68 bits and offers nice API to access things like device ID data, keeping implementation details at arms length from applications. I could also provide a tool that takes a descriptor file in a human readable format and converts it into a proper EOMA68 packed structure to be written to an EEPROM.
awesome!
Obviously we need agreement on the above before I do this.
yes. my only main addition would be that "capabilities" field, as well as asking you to come up with some reasonable argument as to why the risk of vendors (or viruses) overwriting critical data should be considered.
Some additional semi-random notes from reading the EOMA68 spec regarding the EEPROM:
In the "Start-up procedure” section, it gives the EEPROM address as 0xA2 rather than 0x51. I would recommend not putting the address here, but rather dealing with all those details in the "Requirements for I2C” section and giving the EEPROM a proper name there (e.g. the RID EEPROM, for Required IDentification).
... ey good idea.
In fact, I’d re-org that section entirely to be like:
Required Interfaces Rational for interface selections* i2c The "Required IDentification” EEPROM USB SATA ETHERNET RGB/TTL
- I would remove all mention of specific vendors here; this is a specification
after all,
true.
so I’d kill the last para about All Winner and costing. I would also move each of the line items about the interfaces to the section on that interface
Thoughts?
register on elinux.org, knock yourself out. it's a wiki after all :) let's see what happens. i really appreciate someon else being willing to contribute to the development process in such a large way that helps make the spec "real".
l.