Re: [Embedded-pv-devel] [PATCH v10] xen: add para-virtual sound interface header file

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

>>>>>> +struct xensnd_close_req {
>>>>>> +};
>>>>>
>>>>>
>>>>>
>>>>> I'm afraid structures without any members aren't permitted by C89.
>>>>>
>>>> There are 2 options I see:
>>>> 1. Remove empty structures
>>>> 2. Leave them in, but add a comment and add a dummy place holder
>>>> I would prefer option 1 as this way the interface seems to be clearer,
>>>> like all requests are defined etc.
>>
>> Typo: I would prefer option 2 with placeholders and comments
>>>>>>
>>>>>>
>>>>>> +struct xensnd_request {
>>>>>> +     uint8_t raw[64];
>>>>>> +};
>>>>>> +
>>>>>> +struct xensnd_response {
>>>>>> +     uint8_t raw[64];
>>>>>> +};
>>>>>
>>>>>
>>>>>
>>>>> What are these two needed for now? If you want to enforce a
>>>>> minimum size, that would better go ...
>>>>>
>>>> That is because we want to be cache line aligned.
>>>
>>>
>>>
>>> But this is only accurate to your platform. There is HW available with
>>> 128
>>> bytes.
>>>
>>> For instance on Xen ARM, all the structures are aligned to 128 bytes in
>>> order to fit in most of cache line. Although technically it would be
>>> possible have bigger one (see the definition of CSSIDR).
>>>
>> So, then I will align to 32
>
>
> To confirm, you don't plan to reduce the size of the command packet to 32
> octet?
>
Well, actually this is what I was going to do...
So, what is the correct value to be used here?
>>>> What it means in practice
>>>> is not clear as we have so many HW around, so it is not possible to
>>>> fit all. So the options could be:
>>>> 1. 32 or 64
>>>> 2. 48 I think is not an option here
>>>
>>>
>>>
>>> Why 48?
>>
>> It is not an option, saw something like this in fbif.h, but it is 40
>> actually there:
>> #define XENFB_OUT_EVENT_SIZE 40
>
>
> But your 32 value seems to be as random as 40. I guess they define a size
> that would fit all the command and would potentially allow the addition of
> commands.
>
Yes, you are right here
> --
> Julien Grall
Best regards,
Oleksandr Andrushchenko

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