[PATCH] fdtdec: drop needlessly convoluted CONFIG_PHANDLE_CHECK_SEQ

[Rasmus Villemoes]

On 19/05/2022 12.41, Aswath Govindraju wrote:
> Hi Rasmus,
> 
> On 19/05/22 14:40, Rasmus Villemoes wrote:
>> Asking if the alias we found actually points at the device tree node
>> we passed in (in the guise of its offset from blob) can be done simply
>> by asking if the fdt_path_offset() of the alias' path is identical to
>> offset.
>>
>> In fact, the current method suffers from the possibility of false
>> negatives: dtc does not necessarily emit a phandle property for a node
>> just because it is referenced in /aliases; it only emits a phandle
>> property for a node if it is referenced in <angle brackets>
>> somewhere. So if both the node we passed in and the alias node we're
>> considering don't have phandles, fdt_get_phandle() returns 0 for both.
>>
>> Since the proper check is so simple, there's no reason to hide that
>> behind a config option (and if one really wanted that, it should be
>> called something else because there's no need to involve phandle in
>> the check).
>>
>> Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
>> ---
>>  configs/am65x_evm_a53_defconfig  | 1 -
>>  configs/evb-ast2600_defconfig    | 1 -
>>  configs/sama7g5ek_mmc1_defconfig | 1 -
>>  configs/sama7g5ek_mmc_defconfig  | 1 -
>>  lib/Kconfig                      | 7 -------
>>  lib/fdtdec.c                     | 7 ++-----
>>  6 files changed, 2 insertions(+), 16 deletions(-)
>>
>> diff --git a/configs/am65x_evm_a53_defconfig b/configs/am65x_evm_a53_defconfig
>> index 9f41b397c3..a635d6d137 100644
>> --- a/configs/am65x_evm_a53_defconfig
>> +++ b/configs/am65x_evm_a53_defconfig
>> @@ -170,4 +170,3 @@ CONFIG_USB_GADGET_VENDOR_NUM=0x0451
>>  CONFIG_USB_GADGET_PRODUCT_NUM=0x6162
>>  CONFIG_USB_GADGET_DOWNLOAD=y
>>  CONFIG_OF_LIBFDT_OVERLAY=y
>> -CONFIG_PHANDLE_CHECK_SEQ=y
>> diff --git a/configs/evb-ast2600_defconfig b/configs/evb-ast2600_defconfig
>> index ea75762926..015df20f09 100644
>> --- a/configs/evb-ast2600_defconfig
>> +++ b/configs/evb-ast2600_defconfig
>> @@ -84,4 +84,3 @@ CONFIG_WDT=y
>>  CONFIG_SHA384=y
>>  CONFIG_HEXDUMP=y
>>  # CONFIG_EFI_LOADER is not set
>> -CONFIG_PHANDLE_CHECK_SEQ=y
>> diff --git a/configs/sama7g5ek_mmc1_defconfig b/configs/sama7g5ek_mmc1_defconfig
>> index 42d96f7c02..3f33eb1142 100644
>> --- a/configs/sama7g5ek_mmc1_defconfig
>> +++ b/configs/sama7g5ek_mmc1_defconfig
>> @@ -76,4 +76,3 @@ CONFIG_TIMER=y
>>  CONFIG_MCHP_PIT64B_TIMER=y
>>  CONFIG_OF_LIBFDT_OVERLAY=y
>>  # CONFIG_EFI_LOADER_HII is not set
>> -CONFIG_PHANDLE_CHECK_SEQ=y
>> diff --git a/configs/sama7g5ek_mmc_defconfig b/configs/sama7g5ek_mmc_defconfig
>> index e03a6ba9af..6266eb0b59 100644
>> --- a/configs/sama7g5ek_mmc_defconfig
>> +++ b/configs/sama7g5ek_mmc_defconfig
>> @@ -76,4 +76,3 @@ CONFIG_TIMER=y
>>  CONFIG_MCHP_PIT64B_TIMER=y
>>  CONFIG_OF_LIBFDT_OVERLAY=y
>>  # CONFIG_EFI_LOADER_HII is not set
>> -CONFIG_PHANDLE_CHECK_SEQ=y
>> diff --git a/lib/Kconfig b/lib/Kconfig
>> index acc0ac081a..884569f9b1 100644
>> --- a/lib/Kconfig
>> +++ b/lib/Kconfig
>> @@ -958,11 +958,4 @@ config LMB_RESERVED_REGIONS
>>  	  Define the number of supported reserved regions in the library logical
>>  	  memory blocks.
>>  
>> -config PHANDLE_CHECK_SEQ
>> -	bool "Enable phandle check while getting sequence number"
>> -	help
>> -	  When there are multiple device tree nodes with same name,
>> -          enable this config option to distinguish them using
>> -	  phandles in fdtdec_get_alias_seq() function.
>> -
>>  endmenu
>> diff --git a/lib/fdtdec.c b/lib/fdtdec.c
>> index e20f6aad9c..ffa78f97ca 100644
>> --- a/lib/fdtdec.c
>> +++ b/lib/fdtdec.c
>> @@ -516,11 +516,8 @@ int fdtdec_get_alias_seq(const void *blob, const char *base, int offset,
>>  		 * Adding an extra check to distinguish DT nodes with
>>  		 * same name
>>  		 */
>> -		if (IS_ENABLED(CONFIG_PHANDLE_CHECK_SEQ)) {
>> -			if (fdt_get_phandle(blob, offset) !=
>> -			    fdt_get_phandle(blob, fdt_path_offset(blob, prop)))
>> -				continue;
>> -		}
>> +		if (offset != fdt_path_offset(blob, prop))
>> +			continue;
> 
> The offset over here is the offset of the dt node

Yes.

> and the offset that we
> get from fdt_path_offset(blob, prop) is the offset of the aliases
> property. 

No. Here "prop" is the value of the property under the /aliases node
with the name "name", i.e. if you have

    aliases {
        ethernet0 = "/soc at 0/bus at 30800000/ethernet at 30be0000";
    }

then name would be "ethernet0" and "prop" would be
"/soc at 0/bus at 30800000/ethernet at 30be0000". That's why there's some sanity
checks on prop before this, i.e. the value must start with a '/' and
must be a proper nul-terminated string:

	if (len < find_namelen || *prop != '/' || prop[len - 1] ||
	    strncmp(name, base, base_len))
		continue;

The next check then matches the basename of that path against the name
of the node passed in:

	slash = strrchr(prop, '/');
	if (strcmp(slash + 1, find_name))
		continue;

And then fdt_path_offset() simply follows the given path from the root
of blob to that node and returns its offset. [Yes, fdt_path_offset()
also supports non-absolute paths and in that case does an alias lookup
by itself, but we're not passing the alias, we're passing the value of
that alias which is the absolute path.]

> I don't think these both offsets will match for any case. That
> is the reason behind comparing phandles and not offsets.

It does, I've tested it.

> Also, as fdt_path_offset() slow and this would effect the U-Boot
> startup.

First, we only get to do that fdt_path_offset() if the node names
actually match, so it's not for each and every alias lookup. Second, as
I pointed out, it's somewhat fragile to rely on (at least one of) the
nodes in question to even have a phandle.

> To avert the time penalty on all boards, this extra check
> put under a config option.

I prefer correctness out-of-the-box instead of having to discover some
config knob, while also somehow making sure that all nodes do get
phandles. And since this no longer does two fdt_get_phandle() calls, the
overhead is much smaller. I really doubt you can measure any difference
in boot time. If you can, let's add a config option, but make it opt-out
with a help text that says "only disable this if you know you never have
two nodes with the same node name".

Rasmus