Re: [PATCH v4] arm: Add Kconfig entry to select CONFIG_DTB_FILE

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.03.2021 13:39, Michal Orzel wrote:
> On 11.03.2021 12:11, Julien Grall wrote:
>> On 11/03/2021 10:41, Michal Orzel wrote:
>>> On 11.03.2021 11:34, Julien Grall wrote:
>>>> On 10/03/2021 06:58, Michal Orzel wrote:
>>>>> Currently in order to link existing DTB into Xen image
>>>>> we need to either specify option CONFIG_DTB_FILE on the
>>>>> command line or manually add it into .config.
>>>>> Add Kconfig entry: CONFIG_DTB_FILE to be able to
>>>>> provide the path to DTB we want to embed into Xen image.
>>>>> If no path provided - the dtb will not be embedded.
>>>>>
>>>>> Remove the line: AFLAGS-y += -DCONFIG_DTB_FILE=\"$(CONFIG_DTB_FILE)\"
>>>>> as it is not needed since Kconfig will define it in a header
>>>>> with all the other config options.
>>>>>
>>>>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>>>>> ---
>>>>>    xen/arch/arm/Makefile | 5 ++---
>>>>>    xen/common/Kconfig    | 8 ++++++++
>>>>>    2 files changed, 10 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
>>>>> index 16e6523e2c..46e6a95fec 100644
>>>>> --- a/xen/arch/arm/Makefile
>>>>> +++ b/xen/arch/arm/Makefile
>>>>> @@ -68,9 +68,8 @@ extra-y += $(TARGET_SUBARCH)/head.o
>>>>>      #obj-bin-y += ....o
>>>>>    -ifdef CONFIG_DTB_FILE
>>>>> +ifneq ($(CONFIG_DTB_FILE),"")
>>>>>    obj-y += dtb.o
>>>>> -AFLAGS-y += -DCONFIG_DTB_FILE=\"$(CONFIG_DTB_FILE)\"
>>>>>    endif
>>>>>      ALL_OBJS := $(TARGET_SUBARCH)/head.o $(ALL_OBJS)
>>>>> @@ -137,7 +136,7 @@ asm-offsets.s: $(TARGET_SUBARCH)/asm-offsets.c
>>>>>    xen.lds: xen.lds.S
>>>>>        $(CPP) -P $(a_flags) -MQ $@ -o $@ $<
>>>>>    -dtb.o: $(CONFIG_DTB_FILE)
>>>>> +dtb.o: $(patsubst "%",%,$(CONFIG_DTB_FILE))
>>>>>      .PHONY: clean
>>>>>    clean::
>>>>> diff --git a/xen/common/Kconfig b/xen/common/Kconfig
>>>>> index eb953d171e..a27836bf47 100644
>>>>> --- a/xen/common/Kconfig
>>>>> +++ b/xen/common/Kconfig
>>>>> @@ -400,6 +400,14 @@ config DOM0_MEM
>>>>>            Leave empty if you are not sure what to specify.
>>>>>    +config DTB_FILE
>>>>> +    string "Absolute path to device tree blob"
>>>>> +    depends on HAS_DEVICE_TREE
>>>>> +    ---help---
>>>>> +      When using a bootloader that has no device tree support or when 
>>>>> there
>>>>> +      is no bootloader at all, use this option to specify the absolute 
>>>>> path
>>>>> +      to a device tree that will be linked directly inside Xen binary.
>>>>
>>>> With this approach, CONFIG_DTB_FILE will always be defined. This means 
>>>> that Xen will always be compiled to use the "embedded" DTB. When the 
>>>> string is "", it will be garbagge.
>>>>
>>>> So I think we need a second config to that indicates whether the string is 
>>>> empty or not.
>>>>
>>>> Interestingly, your first version of patch didn't expose the problem 
>>>> because CONFIG_DTB_FILE would not be defined if the CONFIG_LINK_DTB is not 
>>>> selected. Although, it would still happily build if CONFIG_DTB_FILE is "".
>>>>
>>>> Cheers,
>>>>
>>> I do not agrree. We do not need another config.
>>
>> Did you test that Xen will still boot if the string is empty?
>>
>>> If string is empty - the dtb.o will not be created and there will be no dtb 
>>> section in xen binary.
>>
>> The dtb.o will not be created but the section will because the linker use 
>> #ifdef CONFIG_DTB_FILE:
>>
>> 42sh> grep CONFIG_DTB .config
>> CONFIG_DTB_FILE=""
>> 42sh> nm xen-syms | grep _sdtb
>> 00000000003560f8 B _sdtb
>>
>> And to show this is going to be used:
>>
>> 42sh> git diff
>> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
>> index 5d44667bd89d..2b680b8226d2 100644
>> --- a/xen/arch/arm/arm64/head.S
>> +++ b/xen/arch/arm/arm64/head.S
>> @@ -297,6 +297,7 @@ real_start_efi:
>>
>>          /* Using the DTB in the .dtb section? */
>>  #ifdef CONFIG_DTB_FILE
>> +        e
>>          load_paddr x21, _sdtb
>>  #endif
>>
>> 42hs> make
>> [...]
>>   CC      arm64/head.o
>> arm64/head.S: Assembler messages:
>> arm64/head.S:300: Error: unknown mnemonic `e' -- `e'
>> /home/ANT.AMAZON.COM/jgrall/works/oss/xen/xen/Rules.mk:204: recipe for 
>> target 'arm64/head.o' failed
>>
>> So _sdtb is going to always be used...
>>
>> Cheers,
>>
> 
> Yes you are right. So I could add another config like:
> config DTB_VALID
>       def_bool y if DTB_FILE != ""
> and change all the lines containing:
> #ifdef CONFIG_DTB_FILE
> to
> #ifdef CONFIG_DTB_VALID

I'm sorry to jump in again, but I still think a 2nd Kconfig setting
is not needed. I count three uses of CONFIG_DTB_VALID outside of
make files. The ones in .S can be replaced by using assembler
directives .ifeqs / .ifneqs. And the one in xen.lds.S looks to be
unnecessary altogether: If there's no input .dtb section, the
linker wouldn't allocate an output one anyway. And the _sdtb symbol,
if you want to avoid its creation when there's no reference, could
be wrapped in PROVIDE(). (I also think that symbol should be
defined inside the section definition, not ahead of it.)

Jan