Re: guidance on struct alignment for rtl8192cu driver

[Larry Finger <Larry.Finger@lwfinger.net>]

On 09/16/2013 09:35 AM, Seth Forshee wrote:
> On Sat, Sep 14, 2013 at 09:08:34AM -0500, Larry Finger wrote:
>> On 09/14/2013 12:36 AM, Jason Andrews wrote:
>>> I'm using an ASUS USB N13 on an ARM platform with the rtl8192cu driver.
>>> Linux kernel is 3.10 so I probably don't have the latest and greatest driver.
>>>
>>> When I booted I got an ARM alignment trap caused by the driver.
>>>
>>> I determined the cause was the 1st argument to spin_lock_irqsave() has an unaligned address.
>>>
>>> By trial-and-error I found that if I edit wifi.h and insert 2 dummy bytes into the rtl_priv struct just above priv (last variable) the locks work and the driver works fine.
>>>
>>> What is the recommended way to make sure the last variable in the rtl_priv struct (u8 priv[0]) is aligned on a 4 byte boundary so the driver works on ARM machines?
>>
>> There are a lot of improvements for this driver in 3.11. The
>> backports release has that code. In addition, I am currently working
>> at improving the power management for 3.13.
>>
>> The presence of unaligned variables that cause alignment traps on
>> ARM does not surprise me as I test only on x86 and ppc
>> architectures. I now own a Raspberry Pi and I will soon be testing
>> with it as well.
>>
>> What does surprise me is that the first argument in all the calls to
>> spin_lock_irqsave() are contained within the rtl_locks struct and
>> everything there should be aligned. Perhaps some ARM expert will
>> know why aligning the last item in the rtl_priv struct fixes the
>> problem.
>
> Depending on architecture version and configuration ARM may or may not
> allow unaligned accesses. Even when allowed there is a cost though, so
> it's better to properly align the data. In the past this would have
> always meant 4-byte alignment, but my ARM experience is a bit dated now
> and I don't know about 64-bit ARM. That variable-size array probably
> only has byte alignment.
>
>> As far as I know, the proper way to do a 4-byte alignment is as in
>> the following patch:
>>
>> Index: wireless-testing-save/drivers/net/wireless/rtlwifi/wifi.h
>> ===================================================================
>> --- wireless-testing-save.orig/drivers/net/wireless/rtlwifi/wifi.h
>> +++ wireless-testing-save/drivers/net/wireless/rtlwifi/wifi.h
>> @@ -2057,7 +2057,7 @@ struct rtl_priv {
>>   	  that it points to the data allocated
>>   	  beyond  this structure like:
>>   	  rtl_pci_priv or rtl_usb_priv */
>> -	u8 priv[0];
>> +	u8 __aligned(4) priv[0];
>>   };
>
> __attribute__((aligned)) might be a safer bet, as this will align it to
> the largest alignment that could possibly be needed.

Seth,

Thanks for the help. So far, I have not heard if my original patch helps or not. 
When, and if, I hear, I will use your suggestion for the final patch.

Larry