linux-kernel - Re: [PATCH 1/9] dt: Handle passed/built-in DT selection in early_init_dt

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Date:	Thu, 21 Nov 2013 12:23:44 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Rob Herring <rob.herring@...xeda.com>
Cc:	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/9] dt: Handle passed/built-in DT selection in early_init_dt_scan()

On Thu, Nov 21, 2013 at 12:21 PM, Grant Likely
<grant.likely@...retlab.ca> wrote:
> On Tue, 19 Nov 2013 12:12:26 +0100, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> Let early_init_dt_scan() fall-back to the built-in DT if no DT was passed,
>> or if it's invalid, so architectures don't have to duplicate this logic.
>>
>> Suggested-by: Rob Herring <rob.herring@...xeda.com>
>> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> ---
>>  drivers/of/fdt.c       |   24 +++++++++++++++++-------
>>  include/linux/of_fdt.h |    1 +
>>  2 files changed, 18 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
>> index 2fa024b97c43..a797cd43bc8b 100644
>> --- a/drivers/of/fdt.c
>> +++ b/drivers/of/fdt.c
>> @@ -868,18 +868,28 @@ void * __init __weak early_init_dt_alloc_memory_arch(u64 size, u64 align)
>>
>>  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
> memory. The dtb section can also potentially contain multiple .dtb
> blobs. In the use case that you care about you are probably only
> 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.
>
> Instead of using __dtb_start, it would probably solve both of the above
> problems if the build-in DTB has a specific label identifying it as the
> default DT.

Er, not quite. It still doesn't solve the init section discard
problem. That still needs solving.

g.
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