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 22:19, Michael S. Tsirkin wrote:
> On Tue, Jun 16, 2015 at 09:33:19AM +0800, Shannon Zhao wrote:
>>
>>
>> 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).
> 
> With a struct you have a bunch of reserved fields - is that very
> different?
> 

Not only about the reserved fields, but also the fields which ARM
doesn't use or x86 doesn't use. For example, xpm1a_event_block in struct
AcpiFadtDescriptorRev5_1 is not used for ARM now, if we use
build_append_int_noprefix, we should add lots of
build_append_int_noprefix(table, 0, 1/2/4/8). But if we use struct, we
just need to care them when we define it, rather than every time we use.

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

-- 
Shannon