linux-kernel - Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put

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Date:   Sat, 14 Aug 2021 00:07:03 +0800
From:   Xianting TIan <xianting.tian@...ux.alibaba.com>
To:     Jiri Slaby <jirislaby@...nel.org>, gregkh@...uxfoundation.org,
        amit@...nel.org, arnd@...db.de, osandov@...com
Cc:     clang-built-linux@...glegroups.com, kbuild-all@...ts.01.org,
        linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()

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