Re: [PATCH v2] ARM/multiboot: use more flexible node naming

[Andre Przywara <andre.przywara@linaro.org>]

On 09/17/2013 04:53 PM, Ian Campbell wrote:
> On Wed, 2013-09-11 at 16:06 +0200, Andre Przywara wrote:
>> For the current "multiboot" on ARM support we look for a compatible
>> string of "xen,multiboot-module" in the device tree, and then
>> use "xen,linux-zimage" and "xen,linux-initrd" to differentiate
>> between the two supported module types.
>> To meet the more generic multiboot proposal in the device tree [1],
>> allow Xen to be more flexible in the compatible naming and also use
>> the new generic base name "boot,module".
>> The mapping to either Dom0 kernel or RAM disk works by providing a
>> more specific name ("xen,dom0-kernel" and "xen,ramdisk", preferably).
>> For compatibility reasons the older names above are still recognized.
>>
>> Changes from v1:
>> * whitespace / coding style fixes (sorry for that mess!)
>> * removed module enumeration by using module@address
>>    (this violates the EPAPR device tree spec).
>> * added __initconst to names array
>>
>> [1] http://lists.xen.org/archives/html/xen-devel/2013-09/msg00083.html
>>
>> Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
>> ---
>>   xen/common/device_tree.c | 44 +++++++++++++++++++++++++++++++++++++-------
>>   1 file changed, 37 insertions(+), 7 deletions(-)
>>
>> diff --git a/xen/common/device_tree.c b/xen/common/device_tree.c
>> index eed77ce..3ae593f 100644
>> --- a/xen/common/device_tree.c
>> +++ b/xen/common/device_tree.c
>> @@ -433,22 +433,50 @@ static void __init process_cpu_node(const void *fdt, int node,
>>       cpumask_set_cpu(start, &cpu_possible_map);
>>   }
>>
>> +static const char * const __initconst kernel_module_names[] = {
>> +    "xen,linux-zimage",
>> +    "xen,dom0-kernel",
>> +    "boot,kernel",
>
> I'm wondering about this..
>
> The current "xen,linux-zimage" node does more than simply identifying
> the location in memory of the kernel, it actually tells us that the boot
> protocol used by that kernel is the Linux zImage protocol.
>
> For "xen,dom0-kernel" and "boot,kernel" we don't get that -- so how do
> we know how the kernel in question wants to be called?

Good point. Yes, we could fix this by looking for magics (ARM zImage has 
one at 0x24, along with start and end address).
Hopefully BSD has something similar or we have to use it the lack of a 
magic to detect those kernels.

How do we solve this on x86? Is that because the Dom0 kernel is PV there 
that we know how to boot this? After all Xen just sees a multiboot 
module, which is actually nothing more like a binary blob in that protocol.
Actually I modeled this along the x86 scheme: Multiboot provides no 
further information about the image other than an order (first is 
kernel, second is initrd). So as this order thing does not work in DT, I 
wanted to use compatible strings to at least determine the type of binary.

> I guess there is a magic number in the kernel itself, so maybe this is
> OK? Any futyre kernel (I'm thinking *BSD here...) would either need to
> be identifiable by magic number or use only a specific compatible value
> which indicates this...

Right, my idea was just to do that: use specific compatible values 
whenever possible or hope for support in Xen to detect the right way of 
booting. So the early mapping code could just live with boot,dom0-kernel 
or even boot,module without knowing anything about a specific boot 
protocol at all. Later on when we actually launch the kernel, we use 
more specific values to pick the right protocol.
Kind of moot discussion probably until we actually have a "second" 
kernel for real.

> OK, I think I've convinced myself, I'll leave the commentary above in
> case you can spot a flaw in my reasoning.
>
>> +    NULL
>> +};
>> +
>> +static const char * const __initconst initrd_module_names[] = {
>> +    "xen,linux-initrd",
>> +    "xen,ramdisk",
>> +    "boot,ramdisk",
>> +    NULL
>> +};
>> +
>>   static void __init process_multiboot_node(const void *fdt, int node,
>>                                             const char *name,
>>                                             u32 address_cells, u32 size_cells)
>>   {
>>       const struct fdt_property *prop;
>>       const u32 *cell;
>> -    int nr;
>> +    int nr = -1;
>>       struct dt_mb_module *mod;
>>       int len;
>> +    const char* const * name_list;
>>
>> -    if ( fdt_node_check_compatible(fdt, node, "xen,linux-zimage") == 0 )
>> -        nr = MOD_KERNEL;
>> -    else if ( fdt_node_check_compatible(fdt, node, "xen,linux-initrd") == 0)
>> -        nr = MOD_INITRD;
>> -    else
>> -        early_panic("%s not a known xen multiboot type\n", name);
>> +    for ( name_list = kernel_module_names; *name_list != NULL; name_list++ )
>> +        if ( fdt_node_check_compatible(fdt, node, *name_list) == 0 )
>
> I've just pushed Julien's big DTB series which adds a helper for
> matching against a list of compatible nodes.
>
>> +        {
>> +            nr = MOD_KERNEL;
>> +            break;
>> +        }
>> +
>> +    for ( name_list = initrd_module_names; *name_list != NULL; name_list++ )
>> +        if ( fdt_node_check_compatible(fdt, node, *name_list) == 0 )
>> +        {
>> +            nr = MOD_INITRD;
>> +            break;
>> +        }
>> +
>> +    if ( nr == -1 )
>> +    {
>> +        early_printk("warning: %s not a known module type, ignoring\n", name);
>> +        return;
>> +    }
>>
>>       mod = &early_info.modules.module[nr];
>>
>> @@ -486,6 +514,8 @@ static int __init early_scan_node(const void *fdt,
>>           process_cpu_node(fdt, node, name, address_cells, size_cells);
>>       else if ( device_tree_node_compatible(fdt, node, "xen,multiboot-module" ) )
>
> Should we mark this as deprecated in the docs?
>
> Speaking of which, this patch doesn't seem to ouch the docs tree ;-)

Actually I thought people would slap me earlier for that ;-)

I wanted to wait until we settle with this general device tree booting 
proposal thing before writing the docs. But on the other hand this is 
just an excuse, so I will respin the patch...

Regards,
Andre.

>
> I know the doc should live somewhere else, but lets try and keep it up
> to date until we move it...
>
>>           process_multiboot_node(fdt, node, name, address_cells, size_cells);
>> +    else if ( device_tree_node_compatible(fdt, node, "boot,module" ) )
>> +        process_multiboot_node(fdt, node, name, address_cells, size_cells);
>>
>>       return 0;
>>   }
>
>