Re: [Qemu-devel] [PATCH v3 2/2] hw/arm/virt-acpi-build: Add SPCR table

[Shannon Zhao <shannon.zhao@linaro.org>]

On 2015/6/16 2:13, Michael S. Tsirkin wrote:
> On Mon, Jun 15, 2015 at 05:59:06PM +0100, Peter Maydell wrote:
>> On 15 June 2015 at 17:32, Andrew Jones <drjones@redhat.com> wrote:
>>> On Mon, Jun 15, 2015 at 06:10:25PM +0200, Michael S. Tsirkin wrote:
>>>> On Mon, Jun 15, 2015 at 04:45:58PM +0100, Peter Maydell wrote:
>>>>> I'm still confused about when fields in these ACPI structs
>>>>> need to be converted to little-endian, and when they don't.
>>>>> Is there a rule-of-thumb I can use when I'm looking at patches?
>>
>>>> Normally it's all LE unless it's a single byte value.
>>>> Did not check this specific table.
>>>> We really need to add sparse support to check
>>>> endian-ness matches, or re-write it
>>>> all using byte_add so there's no duplication of info.
>>
>>> Everything used in the table is either a single byte, or I used le32,
>>> Well, I didn't bother for the pci_{device,vendor}_id assignments, as
>>> they're 0xffff anyway. I can change those two to make them more explicit,
>>> if that's preferred.
>>
>> Yep, I just looked over the struct definition, so since this
>> has been reviewed I'll apply it to target-arm.next.
>>
>> You could probably make it easier to review and write
>> code that has to do these endianness swaps with something
>> like
>>
>> #define acpi_struct_assign(FIELD, VAL) \
>>   ((FIELD) = \
>>   __builtin_choose_expr(sizeof(FIELD) == 1, VAL, \
>>   __builtin_choose_expr(sizeof(FIELD) == 2, cpu_to_le16(VAL), \
>>   __builtin_choose_expr(sizeof(FIELD) == 4, cpu_to_le32(VAL), \
>>   __builtin_choose_expr(sizeof(FIELD) == 8, cpu_to_le64(VAL), \
>>   abort))))
>>
>> (untested, but based on some code in linux-user/qemu.h).
>>
>> Then it's always
>>
>>     acpi_struct_assign(spcr->field, value);
>>
>> whether the field is 1, 2, 4 or 8 bytes.
>>
>> Not my bit of the codebase though, so I'll leave it to the
>> ACPI maintainers to decide how much they like magic macros :-)
>>
>> thanks
>> -- PMM
> 
> 
> We don't much. One can use build_append_int_noprefix and just avoid
> structs altogether.

But if we use build_append_int_noprefix, we have to bother about the
unused fields of the struct and have lots of
build_append_int_noprefix(table, 0, 1/2/4/8).

> We did this for some structures and I'm thinking it's a good direction
> generally.
> 

-- 
Shannon