Hi Simon, On 10/13/21 03:29, Bin Meng wrote: > On Wed, Oct 13, 2021 at 9:01 AM Simon Glass <s...@chromium.org> wrote: >> >> With Ilias' efforts we have dropped OF_PRIOR_STAGE and OF_HOSTFILE so >> there are only three ways to obtain a devicetree: >> >> - OF_SEPARATE - the normal way, where the devicetree is built and >> appended to U-Boot >> - OF_EMBED - for development purposes, the devicetree is embedded in >> the ELF file (also used for EFI) >> - OF_BOARD - the board figures it out on its own >> >> The last one is currently set up so that no devicetree is needed at all >> in the U-Boot tree. Most boards do provide one, but some don't. Some >> don't even provide instructions on how to boot on the board. >> >> The problems with this approach are documented at [1]. >> >> In practice, OF_BOARD is not really distinct from OF_SEPARATE. Any board >> can obtain its devicetree at runtime, even it is has a devicetree built >> in U-Boot. This is because U-Boot may be a second-stage bootloader and its >> caller may have a better idea about the hardware available in the machine. >> This is the case with a few QEMU boards, for example. >> >> So it makes no sense to have OF_BOARD as a 'choice'. It should be an >> option, available with either OF_SEPARATE or OF_EMBED. >> >> This series makes this change, adding various missing devicetree files >> (and placeholders) to make the build work. > > Adding device trees that are never used sounds like a hack to me. > > For QEMU, device tree is dynamically generated on the fly based on > command line parameters, and the device tree you put in this series > has various hardcoded <phandle> values which normally do not show up > in hand-written dts files.
Besides, QEMU generates these dtb at runtime on purpose: it gives emulated machines the freedom to evolve by adding new devices, mapping/wiring peripherals differently. By adding static dtb this gives QEMU users false expectations the machine hardware is stable, or force QEMU to have this interface become a stable API. >From QEMU perspective this seems counter-productive. Digging a bit I see this has already been discussed on qemu-devel@ mailing list recently: https://lore.kernel.org/qemu-devel/CAFEAcA_QNcAHtdxUPLpmyzMYgb9YzhcE0-zyh=n8rqm4voc...@mail.gmail.com/ > I am not sure I understand the whole point of this. > >> >> It also provides a few qemu clean-ups discovered along the way. >> >> This series is based on Ilias' two series for OF_HOSTFILE and >> OF_PRIOR_STAGE removal. >> >> It is available at u-boot-dm/ofb-working >> >> [1] >> https://patchwork.ozlabs.org/project/uboot/patch/20210919215111.3830278-3-...@chromium.org/ >> > > Regards, > Bin >
