Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Xianting TIan <xianting.tian@linux.alibaba.com>]

在 2021/8/13 下午1:53, Jiri Slaby 写道:
> Hi,
>
> On 12. 08. 21, 14:26, kernel test robot wrote:
>>>> drivers/tty/hvc/hvc_console.c:190:26: warning: variable 'hp' is 
>>>> uninitialized when used here [-Wuninitialized]
>>             spin_unlock_irqrestore(&hp->c_lock, flags);
>>                                     ^~
>>     drivers/tty/hvc/hvc_console.c:149:23: note: initialize the 
>> variable 'hp' to silence this warning
>>             struct hvc_struct *hp;
>>                                  ^
>>                                   = NULL
>
> So you clearly didn't test your change as it would crash quite 
> instantly. I wonder, where do you intend to get hp from in the 
> console::print() hook?
>
> thanks,
According to analysis, this issue may can be solved just by adjust the 
alignment to L1_CACHE_BYTES: 

#define __ALIGNED__ __attribute__((__aligned__(L1_CACHE_BYTES)))

Our analysis as below, the original __ALIGNED__ is sizeof(long) which is 
8 for 64bit cpu.
char c[N_OUTBUF] __ALIGNED__; //c[16] __ALIGNED__;

For 4K page, c[16] may cross the page when alignemnt is 8.

In the case the physical address of c[16] is noncontiguous.
|------8----|..........|-----8-----|    PAGE_1
..........................|-----------16----------| c[16]

.........................................|-----8-----|.............|-----8-----| 
PAGE_2
But when the alignment is L1_CACHE_BYTES(eg, 64), or at least 
N_OUTBUF(16), we have no dma issue as c[16] won't cross the page, the 
physical address of c[16] is contiguous.
|--------64--------|.........|--------64---------| PAGE_3

..................................|--c[16]--|

Could you help comments this?  thanks