Re: [Qemu-devel] [PATCHv8 1/3] fw_cfg: switch fw_cfg_find() to locate the fw_cfg device by type rather than path

[Eduardo Habkost]

On Tue, Jul 11, 2017 at 09:02:11PM +0100, Mark Cave-Ayland wrote:
> This will enable the fw_cfg device to be placed anywhere within the QOM tree
> regardless of its machine location.
> 
> Note that we also add a comment to document the behaviour that we return NULL 
> to
> indicate failure where either no fw_cfg device or multiple fw_cfg devices are
> found.
> 
> Signed-off-by: Mark Cave-Ayland <address@hidden>
> ---
>  hw/nvram/fw_cfg.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> index 99bdbc2..8ef889a 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -1017,7 +1017,12 @@ FWCfgState *fw_cfg_init_mem(hwaddr ctl_addr, hwaddr 
> data_addr)
>  
>  FWCfgState *fw_cfg_find(void)
>  {
> -    return FW_CFG(object_resolve_path(FW_CFG_PATH, NULL));
> +    /* Returns FWCfgState if only one fw_cfg device type exists. If zero or
> +       more than one fw_cfg device are found then NULL is returned as per the
> +       object_resolve_path_type() documentation. This behaviour is correct as
> +       it ensures that we detect both missing fw_cfg devices and multiple
> +       fw_cfg devices which could result in unpredictable behaviour. */
> +    return FW_CFG(object_resolve_path_type("", TYPE_FW_CFG, NULL));
>  }

I believe a one-line "returns NULL unless there is exactly one
fw_cfg device" (similar to the one at find_vmgenid_dev()) would
suffice.  :)

-- 
Eduardo