Hi, On Fri, 5 Nov 2021 at 09:51, François Ozog <[email protected]> wrote: > > 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 >
I am pinging this thread again as it has come up on the U-Boot mailing list[1]. Please can you reconsider this patch. It would greatly help the collaboration between QEMU and U-Boot. Regards, Simon [1] https://lore.kernel.org/u-boot/CAFEAcA-TH96CG1gi4toAQXpnxs4kxsPCSSfZrXbo2QF8CoE=x...@mail.gmail.com/T/#m1b41b54e3ca5dda1fcb8fdd189ac472ec7e4a96d
