Modules for carrier boards [Was: Re: Question about extension board used in U-boot]

[Daniel Thompson]

On Sat, Sep 18, 2021 at 08:49:48AM +0200, François Ozog wrote:
> Hi Paul
> 
> Too posting because I think we also need to address this at a higher level.
> 
> i think we discussed this topic quite a while back. I may be wrong but it
> may be Bill Mills who proposed to have an eeprom on the extensions that the
> carrier board can use to detect and fetch proper overlay. Another way would
> be that the contract between the extension board and the carrier board
> includes an i2c accessible storage to fetch an overlay that would identify
> the board and give all details.

What you're describing sounds exactly like Raspberry Pi HATs work:
https://github.com/raspberrypi/hats/blob/master/devicetree-guide.md

Similarly Beagleboard capes use rely on I2C EEPROMs for make them
discoverable, although I don't think all have to have a built-in
overlay (IIRC because they joined the party too early).

In other words there's plenty of prior art here and, as new hardware
standards come out, it should be much easier for them to find this prior
art. However I'm near certain mistakes will still be made...


> Bottom line, a software only solution seems not entirely satisfying.
> In that suboptimal case, U-Boot shall be able to assemble a DT for itself
> and another for OS (may be same in some cases) through scripting. And in
> this case, come your questions  below.

Sub-optimal or not[1] the u-boot extension board code still looks like it
would be a good starting point even for boards with non-discoverable
extensions (96Boards CE 1.0 for example).

If implementing on a board with non-discoverable extensions then I would
consider implementing "extension scan" to report non-discoverable modules
(e.g. from an internal list) and proposing patches to that "extension
apply all" would not enable non-discoverable boards (so that non-
discoverable boards would have to be enabled by injecting a "extension
apply <id>" into the boot scripts).

Of course, I may have overlooked a better existing mechanism in u-boot
but that's what I would start with until I was corrected by
maintainers ;-) .


Daniel.


[1] And also extremely off-topic for Paul since his (a) boards are
    discoverable and (b) the extension framework can't fire up early
    enough for TPM extensions ;-) .



> Le sam. 18 sept. 2021 à 01:21, Ying-Chun Liu (PaulLiu) <paulliu at debian.org>
> a écrit :
> 
> > Hi all,
> >
> >
> > I have some questions about how to implement extension board usage.
> > My case is on imx8mm-cl-iot-gate. It can add three different types of
> > extension boards.
> > One of the extension boards is SPI extension which have 3 empty slots.
> > And you can add
> > some small boards onto it. One of them is a "TPM2" module.
> >
> >
> > My first question is if I want to use tpm2 in U-boot for measured boot.
> > How to implement this right? Currently I just modify the dts used by
> > U-boot to let it drive
> > the extension board. And let it drive the TPM. But it is not good for
> > upstreaming because
> > when other types of extension boards installed then it is not working.
> > Where to implement this? What is the best practice of this?
> 
> 
> > The second question is about extension manager.
> > I have read the extension.rst. I think I'll implement this anyway
> > because then
> > I can have a command to query what type of extension boards I have.
> > And if the extension board is the 3 slots one. I can then detect which
> > slot is the TPM.
> > I'll implement this anyway because the "extension" command is convenient
> > for users.
> > But it seems to me that it only solves the problem for Linux kernel.
> > It can apply a DTB Overlay to Linux DTB to let Linux knows we have that
> > extension board.
> > But it is too late for U-boot itself, right?
> >
> >
> > The third question is I'm also dong SystemReady IR certificate. That means
> > the dtb for Linux is directly provided by U-boot. We use U-boot dtb
> > directly to Linux
> > kernel. In this case, how to modify that dts dynamically to feed to the
> > Linux kernel by
> > the extension manager?
> > What is the best practice if I want to use U-boot dts for Linux in
> > implementation?
> >
> >
> > Thanks a lot.
> >
> >
> > Yours,
> > Paul
> >
> >
> > --
> François-Frédéric Ozog | *Director Business Development*
> T: +33.67221.6485
> francois.ozog at linaro.org | Skype: ffozog
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