[SLOF] [PATCH] libusb: Fix compiler warnings with GCC 9 in the USB code

[Thomas Huth]

On 24/09/2019 11.03, Alexey Kardashevskiy wrote:
> 
> 
> On 20/09/2019 19:06, Thomas Huth wrote:
>> GCC 9 emits a lot of compiler warnings in the USB code like this:
>>
>> [...]
>> usb-ohci.c: In function ‘ohci_get_last_frame’:
>> usb-ohci.c:972:20: warning: taking address of packed member of ‘struct ohci_regs’
>>  may result in an unaligned pointer value [-Waddress-of-packed-member]
>>   972 |  return read_reg32(&regs->fm_num);
>>       |                    ^~~~~~~~~~~~~
>> 	[CC]	usb-ehci.o
>> usb-ehci.c: In function ‘ehci_hub_check_ports’:
>> usb-ehci.c:63:25: warning: taking address of packed member of ‘struct ehci_cap_regs’
>>  may result in an unaligned pointer value [-Waddress-of-packed-member]
>>    63 |  num_ports = read_reg32(&ehcd->cap_regs->hcsparams) & HCS_NPORTS_MASK;
>>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~
>> usb-ehci.c:66:23: warning: taking address of packed member of ‘struct ehci_op_regs’
>>  may result in an unaligned pointer value [-Waddress-of-packed-member]
>>    66 |   portsc = read_reg32(&ehcd->op_regs->portsc[i]);
>>       |                       ^~~~~~~~~~~~~~~~~~~~~~~~~
>> [...]
>> usb-xhci.c:227:20: warning: taking address of packed member of ‘struct xhci_op_regs’
>>  may result in an unaligned pointer value [-Waddress-of-packed-member]
>>   227 |  xhci_wait_for_cnr(&op->usbsts);
>>       |                    ^~~~~~~~~~~
>> usb-xhci.c: In function ‘xhci_poll_event’:
>> usb-xhci.c:309:14: warning: taking address of packed member of ‘struct xhci_int_regs’
>>  may result in an unaligned pointer value [-Waddress-of-packed-member]
>>   309 |  write_reg64(&xhcd->run_regs->irs[0].erdp, val);
>>       |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> [...]
>>
>> Most of the structures are naturally aligned, so we can simply drop the
>> "packed" attribute and use a _Static_assert() instead to make sure that
>> there is no padding here.
>>
>> The only struct that needed some more attention is struct ehci_cap_regs.
>> The portroute field is not aligned to a natural boundary here. But
>> looking at the EHCI spec, it seems like this field is not meant as
>> a full 64-bit value anyway, but rather an array of nibbles, so switch
>> it to an array instead, then we can also drop the "packed" attribute
>> here.
> 
> This one is harder to fix properly (if I knew what "properly" means
> here). I think that for this one the right thing seems to be disabling
> -Waddress-of-packed-member warning whatsoever. May be add an assert for
> hcidev->base alignment.
> 
> Adding asserts as below will silence gcc but won't protect us against
> (very unlikely to happen but nevertheless) unaligned pointers.

I disagree. Disabling -Waddress-of-packed-member will silence the
warning, but won't protect us from unaligned pointers. With my patch,
the structs should be naturally aligned all the time.

 Thomas