Re: [Qemu-arm] [Qemu-devel] [RESEND RFC 4/6] device_tree: introduce qemu_fdt_getprop_optional

[Peter Crosthwaite]

On Thu, Nov 19, 2015 at 7:22 AM, Eric Auger <address@hidden> wrote:
> Current qemu_fdt_getprop exits if the property is not found. It is
> sometimes needed to read an optional property, in which case we do
> not wish to exit but simply returns a null value.
>
> This is what this new qemu_fdt_getprop_optional function does.
>
> Signed-off-by: Eric Auger <address@hidden>
> ---
>  device_tree.c                | 17 +++++++++++++++++
>  include/sysemu/device_tree.h |  2 ++
>  2 files changed, 19 insertions(+)
>
> diff --git a/device_tree.c b/device_tree.c
> index f184e3c..a318683 100644
> --- a/device_tree.c
> +++ b/device_tree.c
> @@ -280,6 +280,23 @@ const void *qemu_fdt_getprop(void *fdt, const char 
> *node_path,
>      return r;
>  }
>
> +const void *qemu_fdt_getprop_optional(void *fdt, const char *node_path,
> +                             const char *property, bool optional, int *lenp)
> +{
> +    int len;
> +    const void *r;
> +    if (!lenp) {
> +        lenp = &len;
> +    }
> +    r = fdt_getprop(fdt, findnode_nofail(fdt, node_path), property, lenp);
> +    if (!r && !optional) {
> +        error_report("%s: Couldn't get %s/%s: %s", __func__,
> +                     node_path, property, fdt_strerror(*lenp));
> +        exit(1);
> +    }
> +    return r;
> +}
> +

The real problem here is that the device-tree API is self-asserting.
This looks the old _nofail system that we removed but just named in
reverse. The correct solution here is to use the Error API properly.
Convert qemu_fdt_getprop to accept an Error **, and all existing users
are converted to pass &error_fatal. This will preserve existing
behaviour. Then to use the API with your optional semantic your pass
NULL for the Error **.

Regards,
Peter

>  uint32_t qemu_fdt_getprop_cell(void *fdt, const char *node_path,
>                                 const char *property)
>  {
> diff --git a/include/sysemu/device_tree.h b/include/sysemu/device_tree.h
> index f9e6e6e..10cbe8e 100644
> --- a/include/sysemu/device_tree.h
> +++ b/include/sysemu/device_tree.h
> @@ -34,6 +34,8 @@ int qemu_fdt_setprop_phandle(void *fdt, const char 
> *node_path,
>                               const char *target_node_path);
>  const void *qemu_fdt_getprop(void *fdt, const char *node_path,
>                               const char *property, int *lenp);
> +const void *qemu_fdt_getprop_optional(void *fdt, const char *node_path,
> +                             const char *property, bool optional, int *lenp);
>  uint32_t qemu_fdt_getprop_cell(void *fdt, const char *node_path,
>                                 const char *property);
>  uint32_t qemu_fdt_get_phandle(void *fdt, const char *path);
> --
> 1.8.3.2
>
>