On Fri, 15 Jul 2022 21:28:57 -0500 Samuel Holland <samuel@xxxxxxxxxxxx> wrote: Hi, > On 7/15/22 7:20 AM, Andre Przywara wrote: > >>>> However, when the board is designed for a specific kind of device which > >>>> is not always present, and the kernel can detect the device, it is > >>>> perfectly fine to describe it. > >>>> > >>>> The alternative is to not use the device at all, even when present, > >>>> which is kind of useless. > >>> > >>> Or let the bootloader update your device tree and disable the device > >>> if it's not there? > > > > Yes, this is what I was suggesting already: U-Boot can do the job, because > > a U-Boot build is device specific, and we can take certain risks that the > > generic and single-image kernel wants to avoid. > > In this case we know that there is a SPI flash footprint, and it does no > > harm in trying to check on CS0. So I was thinking about introducing a > > U-Boot Kconfig variable to probe for and potentially disable the SPI flash > > DT node. We would set this variable in defconfigs of boards with optional > > SPI flash. > > To support the "does no harm" assertion: the Allwinner Boot ROM will probe for > NOR flash (and possibly SPI NAND) on SPI0 + CS0 if no bootable MMC device is > found. So the board designer must already assume that JEDEC Read ID commands > will be sent over that bus. > > >> But then it must be in the device tree? > > > > However this indeed means that the SPI flash DT node must be in and enabled > > in the DT, because we (try hard to) only use original Linux DT files, and > > DTs must have been reviewed through the kernel ML first. The U-Boot driver > > relies on the DT as well, so the official kernel DT copy would need to come > > with that node enabled. Ideally U-Boot would disable it, if needed, and > > the kernel error message would never appear. > > I don't think this works for newer SoCs where the Boot ROM supports both NOR and > SPI NAND. If the board is sold with the flash chip unpopulated, the user could > install either kind of chip. So we cannot statically assume which binding will > be used. We would need to add a node with the right compatible string after > probing for both in the bootloader. If a *user* decides to *change* the board, it's up to the user to make sure the DT matches. Overlays are the typical answer, or people change the DT before they build U-Boot. If someone decides to hack their board, they have to take care of the respective DT description hack as well. This case here is about the *vendor* shipping different versions of the board, which I think is a different case. Technically we now would need two DTs: one with, one without the SPI flash node, and let the user decide which one to include in U-Boot at build time, depending on which version they have. What I was suggesting is a U-Boot config switch, which would only be enabled on those boards where we have this situation (OPi Zero): That avoids dangerous situations (because we know it's safe *on this particular board*), and avoids the hassle of shipping two firmware versions. Cheers, Andre
