Hi Rasmus,
On 19/05/22 16:58, Rasmus Villemoes wrote:
> 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.villem...@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@0/bus@30800000/ethernet@30be0000";
> }
>
> then name would be "ethernet0" and "prop" would be
> "/soc@0/bus@30800000/ethernet@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.]
>
Understood, my bad I stand corrected.
>> 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".
>
Understood, thanks for the explanation. I am good with this patch.
Acked-by: Aswath Govindraju <a-govindr...@ti.com>
--
Thanks,
Aswath
> Rasmus
