Re: [Qemu-devel] [PATCH 4/6] device_tree: qemu_fdt_getprop converted to use the error API

[Eric Auger <eric.auger@linaro.org>]

Hi Peter,
On 12/18/2015 03:36 PM, Peter Maydell wrote:
> On 17 December 2015 at 12:29, Eric Auger <eric.auger@linaro.org> 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 patch converts qemu_fdt_getprop to accept an Error **, and existing
>> users are converted to pass &error_fatal. This preserves the existing
>> behaviour. Then to use the API with your optional semantic a null
>> parameter can be conveyed.
>>
>> Signed-off-by: Eric Auger <eric.auger@linaro.org>
>>
>> ---
>>
>> RFC -> v1:
>> - get rid of qemu_fdt_getprop_optional and implement Peter's suggestion
>>   that consists in using the error API
> 
> This doesn't seem to me like a great way for qemu_fdt_getprop to
> report "property not found", because there's no way for the caller
> to distinguish "property not found" from "function went wrong
> some other way" (since Errors just report human readable strings,
> and in any case you're not distinguishing -FDT_ERR_NOTFOUND
> from any of the other FDT errors).
Not sure I get what you mean here. In case fdt_getprop fails, as long as
the caller provided a lenp != NULL, *lenp contains the error code so
qemu_fdt_getprop's caller can discriminate a -FDT_ERR_NOTFOUND from any
other errors. Do I miss something?
> 
> If we want to handle "ok if property doesn't exist" then we
> could either (a) make the function return NULL on doesn't-exist
> and error_report in the other error cases, with the existing
> single caller extending its error checking appropriately, or
> (b) have the caller that cares about property-may-not-exist
> call fdt_getprop() directly.
> 
>> Signed-off-by: Eric Auger <eric.auger@linaro.org>
>> ---
>>  device_tree.c                | 11 ++++++-----
>>  include/sysemu/device_tree.h |  3 ++-
>>  2 files changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/device_tree.c b/device_tree.c
>> index b5d7e0b..c3720c2 100644
>> --- a/device_tree.c
>> +++ b/device_tree.c
>> @@ -331,18 +331,18 @@ int qemu_fdt_setprop_string(void *fdt, const char *node_path,
>>  }
>>
>>  const void *qemu_fdt_getprop(void *fdt, const char *node_path,
>> -                             const char *property, int *lenp)
>> +                             const char *property, int *lenp, Error **errp)
>>  {
>>      int len;
>>      const void *r;
>> +
>>      if (!lenp) {
>>          lenp = &len;
>>      }
>>      r = fdt_getprop(fdt, findnode_nofail(fdt, node_path), property, lenp);
>>      if (!r) {
>> -        error_report("%s: Couldn't get %s/%s: %s", __func__,
>> -                     node_path, property, fdt_strerror(*lenp));
>> -        exit(1);
>> +        error_setg(errp, "%s: Couldn't get %s/%s: %s", __func__,
>> +                  node_path, property, fdt_strerror(*lenp));
>>      }
>>      return r;
>>  }
>> @@ -351,7 +351,8 @@ uint32_t qemu_fdt_getprop_cell(void *fdt, const char *node_path,
>>                                 const char *property)
>>  {
>>      int len;
>> -    const uint32_t *p = qemu_fdt_getprop(fdt, node_path, property, &len);
>> +    const uint32_t *p = qemu_fdt_getprop(fdt, node_path, property, &len,
>> +                                         &error_fatal);
>>      if (len != 4) {
>>          error_report("%s: %s/%s not 4 bytes long (not a cell?)",
>>                       __func__, node_path, property);
>> diff --git a/include/sysemu/device_tree.h b/include/sysemu/device_tree.h
>> index f9e6e6e..284fd3b 100644
>> --- a/include/sysemu/device_tree.h
>> +++ b/include/sysemu/device_tree.h
>> @@ -33,7 +33,8 @@ int qemu_fdt_setprop_phandle(void *fdt, const char *node_path,
>>                               const char *property,
>>                               const char *target_node_path);
>>  const void *qemu_fdt_getprop(void *fdt, const char *node_path,
>> -                             const char *property, int *lenp);
>> +                             const char *property, int *lenp,
>> +                             Error **errp);
> 
> If we change the function it would be nice to add a brief
> doc comment while we're touching the prototype in the header.
sure

Thanks

Eric
> 
> thanks
> -- PMM
>