Re: [Qemu-devel] [PATCH 07/31] dt: add helper for phandle allocation

[Peter Crosthwaite]

On Thu, Jun 7, 2012 at 2:55 AM, Scott Wood <address@hidden> wrote:
> On 06/06/2012 11:00 AM, Alexander Graf wrote:
>> On 06/06/2012 07:18 AM, Peter Crosthwaite wrote:
>>> On Wed, 2012-06-06 at 01:52 +0200, Alexander Graf wrote:
>>>> Phandle references work by having 2 pieces:
>>>>
>>>>    - a "phandle" 1-cell property in the device tree node
>>>>    - a reference to the same value in a property we want to point
>>>>      to the other node
>>>>
>>>> To generate the 1-cell property, we need an allocation mechanism that
>>>> gives us a unique number space. This patch adds an allocator for these
>>>> properties.
>>>>
>>>> Signed-off-by: Alexander Graf<address@hidden>
>>>> ---
>>>>   device_tree.c |    7 +++++++
>>>>   device_tree.h |    1 +
>>>>   2 files changed, 8 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/device_tree.c b/device_tree.c
>>>> index d4f1f0a..317bdd0 100644
>>>> --- a/device_tree.c
>>>> +++ b/device_tree.c
>>>> @@ -220,6 +220,13 @@ int qemu_devtree_setprop_phandle(void *fdt,
>>>> const char *node_path,
>>>>       return qemu_devtree_setprop_cell(fdt, node_path, property,
>>>> phandle);
>>>>   }
>>>>
>>>> +uint32_t qemu_devtree_alloc_phandle(void *fdt)
>>>> +{
>>>> +    static int phandle = 0x8000;
>>> can easily double check for duplicates. Would also allow you to start
>>> from 1 rather than magic number 0x8000?
>
> You can't check for duplicates, because the tree fragments you'll be
> conflicting with haven't been added to the tree yet.  That's done by a
> tool operating on the tree output by the first pass of qemu, and is fed
> back into the second pass of qemu.
>

Im not sure what you mean by "fragments [which] havent been added to
the tree yet"? Im thinking here of the use model where you use the DTB
API to modify and existing DTB (probably pc-bios/foo.dtb). Yes, for
Alex's series im not sure it wins anything as you are starting a DTB
from scratch (which will have no pre-existing phandles), but it guards
against a potentially very obscure and hard to find bug in some other
potential uses of this API.

Regards,
Peter

> -Scott
>