Hi, On Wed, 3 Nov 2021 at 14:17, François Ozog <[email protected]> wrote:
> Hi, > > Thanks Alex to patch me in. > > I'd like to present another perspective on the motivation as I can't > really comment on the actual "how". > > On real Arm boards, firmware is often assembled into a FIP. > That FIP can contain quite a good deal of things, including an > NT_FW_CONFIG, NonTrusted_FirmWare_CONFIGuration (NT_FW = BL33 which is > U-Boot in our case). > So the expected typical content for that section is a DT fragment/overlay. > That section is to be used in different ways but one is > https://trustedfirmware-a.readthedocs.io/en/latest/components/fconf/index.html > . > For SystemReady systems we will almost inevitably put a device tree > fragment in this section and have BL2 merge it with the board DT before > handing it over to BL33 (U-Boot is one of them). > In some real world examples based on carrier board + som for instance, it > may contain SerDes configuration for U-Boot that will result in appropriate > PCI lanes or MDIO lanes for the booted OS. > > So I could say there is precedence in Simon's effort. > > In any case, when we will have made the changes in TFA for the SystemReady > boards we work on, booting the full SystemReady stack (TFA, OP-TEE, U-Boot) > on Qemu will allow that late merge based through the FIP. > > Other boot flows such as VBE (without TFA but with TPL/SPL/U-Boot proper) > need a similar facility. > > Hence I am supporting Simon's proposal at least on the intent. On the how > exactly, that is outside my skillset. > Responding to my own comment: As the boot flow TFA+U-Boot works, it looks like a cleaner option is to leave QEMU alone and have U-Boot SPL do the same work as with TFA: use a section for the FIT for the DT to be merged. > future comments below > > > On Wed, 3 Nov 2021 at 12:48, Alex Bennée <[email protected]> wrote: > >> >> Peter Maydell <[email protected]> writes: >> >> > On Mon, 27 Sept 2021 at 16:18, Simon Glass <[email protected]> wrote: >> >> On Mon, 27 Sept 2021 at 02:48, Peter Maydell <[email protected]> >> wrote: >> >> > So what is missing in the QEMU-provided DTB that it needs? >> >> >> >> Quite a lot. Here are some examples: >> >> >> >> U-Boot has limited pre-relocation memory so tries to avoid >> >> binding/probing devices that are not used before relocation: >> >> >> >> >> https://u-boot.readthedocs.io/en/latest/develop/driver-model/design.html#pre-relocation-support >> > >> > It's up to u-boot to decide what it wants to touch and >> > what it does not. QEMU tells u-boot what all the available >> > devices are; I don't think we should have extra stuff saying >> > "and if you are u-boot, do something odd". >> > >> >> There is a configuration node (which is likely to change form in >> >> future releases, but will still be there) >> >> >> >> >> https://github.com/u-boot/u-boot/blob/master/doc/device-tree-bindings/config.txt >> > >> > I think u-boot should be storing this kind of thing somewhere >> > else (e.g. as part of the binary blob that is u-boot itself, >> > or stored in flash or RAM as a separate blob). >> > >> >> Then there are various features which put things in U-Boot's control >> >> dtb, such as verified boot, which adds public keys during signing: >> >> >> >> >> https://github.com/u-boot/u-boot/blob/master/doc/uImage.FIT/signature.txt#L135 >> >> >> >> More generally, the U-Boot tree has hundreds of files which add >> >> properties for each board, since we try to keep the U-Boot-specific >> >> things out of the Linux tree: >> >> >> >> $ find . -name *u-boot.dtsi |wc -l >> >> 398 >> > >> > If any of this is actual information about the hardware then you >> > should sort out getting the bindings documented officially >> > (which I think is still in the Linux tree), and then QEMU can >> > provide them. >> > >> >> Quite a bit of this is to do with SPL and so far it seems that QEMU >> >> mostly runs U-Boot proper only, although I see that SPL is starting to >> >> creep in too in the U-Boot CI. >> >> >> >> So at present QEMU is not able to support U-Boot fully. >> > >> > My take is that this is u-boot doing weird custom things with >> > the DTB that aren't "describe the hardware". You should be able >> > to boot u-boot by putting those custom DTB extra things in a >> > separate blob and having u-boot combine that with the >> > actual DTB when it starts. >> >> It's not entirely without precedent - for SPL (which I hope is secondary >> program loading) we have things like the guest loader which expands the >> plain HW DTB with some information needed by the bootloader and the >> primary OS to load additional blobs which have been put into memory. >> >> In effect the DTB is being expanded as a signalling mechanism similar to >> things like fw_cfg and other things we use to control boot up. Whether >> this affects the "purity" of DTB as a "just the HW" description is >> probably a philosophical question. >> >> More than a philosophical question: a key aspect of supply chain that > need change from > quite inflexible and tightly coupled to loosely coupled. > A key aspect of it is to maintain "pure" hardware description DTBs at > rest. > Composition of DTBs at build time (for products) or runtime (for > development boards) should be a simple thing. > Another aspect to take into account is System Device Trees. U-Boot only > deal with Cortex-As on a platform, > so there are multiple device trees for each compute domain. Communities > are working on System Device Tree > to define the overall platform with its power and clock domains. A tool > "lopper" is being developed to slide the SDT into diverse domain DTs. > One of them being included into the FIP as the basis for the computing > element (Carrier, SoM...). > Those attempts to cleanup passed DTBs from configuration data (drivers, > bootloaders...) is not incompatible > with merging fragments at runtime (for dev boards) or build time (for > products). > >> I agree with Peter that just allowing the merging of arbitrary data into >> the QEMU generated DTB is going to lead to confusion and breakages. >> Indeed I wrote the guest-loader because instructions for booting Xen up >> until that point involved dumpdtb and hand hacking the data which was >> silly because this is stuff QEMU already knew about. >> >> > >> > -- PMM >> >> >> -- >> Alex Bennée >> > > > -- > François-Frédéric Ozog | *Director Business Development* > T: +33.67221.6485 > [email protected] | Skype: ffozog > > -- François-Frédéric Ozog | *Director Business Development* T: +33.67221.6485 [email protected] | Skype: ffozog
