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Date: Thu, 21 Nov 2013 15:53:48 +0000 From: Grant Likely <grant.likely@...retlab.ca> To: Geert Uytterhoeven <geert@...ux-m68k.org> Cc: Rob Herring <rob.herring@...xeda.com>, "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, Linux-Arch <linux-arch@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 1/9] dt: Handle passed/built-in DT selection in early_init_dt_scan() On Thu, 21 Nov 2013 14:42:27 +0100, Geert Uytterhoeven <geert@...ux-m68k.org> wrote: > On Thu, Nov 21, 2013 at 1:21 PM, Grant Likely <grant.likely@...retlab.ca> wrote: > >> bool __init early_init_dt_scan(void *params) > >> { > >> - if (!params) > >> - return false; > >> - > >> /* Setup flat device-tree pointer */ > >> initial_boot_params = params; > >> > >> - /* check device tree validity */ > >> - if (be32_to_cpu(initial_boot_params->magic) != OF_DT_HEADER) { > >> - initial_boot_params = NULL; > >> - return false; > >> + /* check passed device tree validity */ > >> + if (initial_boot_params && > >> + be32_to_cpu(initial_boot_params->magic) == OF_DT_HEADER) { > >> + pr_info("FDT at %p\n", initial_boot_params); > >> + goto found; > >> + } > >> + > >> + /* check built-in device tree validity */ > >> + initial_boot_params = &__dtb_start; > >> + if (__dtb_end != (void *)&__dtb_start && > >> + be32_to_cpu(initial_boot_params->magic) == OF_DT_HEADER) { > >> + pr_info("Compiled-in FDT at %p\n", initial_boot_params); > >> + goto found; > > > > __dtb_start is in an init section which is discarded after initcalls. > > For this to work, the dtb needs to be copied into a region of allocated > > My changes don't change the current behavior much: currently > early_init_dt_scan() is already called with &__dtb_start in several places. > If this is broken, it's already broken. Yes, but it is called on platforms that already make that assumption that __dtb_start must be copied and therefore call unflatten_and_copy_devicetree(). This change makes *all* platforms do that. That breaks arm, arm64, c6x, microblaze, some mips platforms, and powerpc! > > memory. The dtb section can also potentially contain multiple .dtb > > blobs. In the use case that you care about you are probably only > > Multiple dtb blobs are currently handled in platform-specific code, which > passes the right dtb to early_init_dt_scan(). The problem is that it makes the default dt completely random because the generic code still deferrences __dtb_start. > > thinking about one, but it is entirely possible for device drivers to > > have a dtb linked in which may break this if it gets linked in a > > different order. The specific example I'm thinking about is I want to > > have the DT selftest code load an overlay to get testcase data from a > > dtb blob. > > > > The other concern I have here is that I don't really want this to be the > > default on a lot of platforms. ARM and PowerPC for instance should only > > get the default dtb from the boot wrapper. It needs to be configurable > > in some way. > > On ARM and PowerPC, the section is empty, hence &__dbt_start == > &__dtb_end. There is no guarantee that that will always be so. The patch makes some poor assumtions, but they shouldn't be difficult to fix. g. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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