On 01/20/2011 03:57 AM, Jacob Avraham wrote: > Hi, > > I have a system with several USB disks, each of them is boot-capable. > Each such device has a kernel and initrd in its filesystem. > I want to be able to determine in initrd from which USB device it (and the > kernel) were > loaded from, so I can mount the root filesystem from that device. > Is that doable? > I don't want to use a static "root=" entry in grub, since I don't know which > sd device it will be.
Your bootloader loads the kernel and hands off control to it. Unless the bootloader informs the kernel where it loaded it from, the kernel has know way of knowing since it was a separate program that did it. (Who says it was a local device? It could have been a PXE boot. It may have been loadlin from Windows that got the kernel off of a network share. It could be the qemu -kernel option that manually populates the virtual system's DRAM with a kernel image and jumps to the start of it when it initializes the virtual CPU.) One of the motivations behind the "device tree" stuff is that the bootloader hands the kernel a standardized data structure telling it where all the hardware is in the system, and one of the things it can annotate this tree with is "and _this_ one was the boot device". Of course both the x86 guys and the ARM maintainer have come out against cross-platform standardization that would require them to change what they're already doing in any way. In general, unless the bootloader tells you where it loaded stuff from, the kernel has no way of knowing. Once upon a time PC hardware had standardized layouts for stuff and the kernel could use heuristics based on knowledge of that to go "ah, first hard drive on the first IDE controller, this partition has the boot flag set, we came from HERE". Then IBM went "this doesn't help us enumerate hardware in our mainframes with 30,000 disks: Linux on the dekstop must accept a standardized mechanism that will inflict the full pain of mainframe device enumeration on the Linux desktop community or this will never be fixed for us!" There hasn't really been much of a Linux desktop community ever since that I've noticed, and device enumeration still sucks, but that's udev for you. Anyway, the kernel can try to guess this info by looking at various devices and seeing if it recognizes any of them. But it's up to you to annotate your devices with with something it can recognize, and programming your initrd to look for that annotation. UUID partition labels are fairly popular. Rob -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
