> On 8 Sep 2016, at 17:34, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 8 Sep 2016, Felipe Balbi wrote:
>
>>>>>>> +struct usb_gadgetfs_features {
>>>>>>> + uint64_t bitmap[4];
>>>>>>
>>>>>> this should really be:
>>>>>>
>>>>>> uint64_t features0;
>>>>>>
>>>>>> /* Keep 24 bytes reserved for possible future usage */
>>>>>> uint8_t RESERVED[24];
>>>>>
>>>>> Isn't this really extreme overkill? I would be surprised if gadgetfs
>>>>> ever supports more than 10 features.
>>>>
>>>> I'm hoping to be extreme overkill :-) I really don't wanna get into a
>>>> situation where we run out of bits. What if some features require
>>>> arguments in the future? We could just pass them here, right?
>>>>
>>>> Frankly, I'm afraid of what happened to i2c-dev :-) We can't change
>>>> (easily) i2c subsystem's write() to return the amount of bytes written
>>>> because its userspace facing ABI has no reserved bits we could use to
>>>> return that value. Changing that would be a huge amount of work.
>>>>
>>>> What do you think?
>>>
>>> Well, I don't care about it all that much, especially since I don't
>>> think you'll ever need to use it. :-)
>>>
>>> Still, there are approaches that have been used in the past when people
>>> wanted to create an extensible interface. Passing a variable-length
>>> structure that has a length field at a reserved spot near the beginning
>>> (perhaps along with one or two other reserved entries), for example.
>>
>> something along the like below?
>>
>>
>> /* better names appreciated */
>> struct usb_gadgetfs_packet {
>> uint16_t length;
>> uint8_t reserved[2];
>>
>> uint8_t *data; /* dynamically allocated by userspace */
>> }
>>
>> I can work with that no problem.
>
> I was thinking more like:
>
> struct usb_gadgetfs_ioctl_arg {
> uint16_t length;
> uint8_t reserved[2];
>
> uint8_t data[0];
> }
>
> but the principle is pretty much the same.
>
> Alan Stern
>
Won’t the user lose the relevant information (e.g. feature structure) by using this model?
Binyamin Sharet
Cisco, STARE-C
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