linux-kernel - Re: [PATCH] MIPS: Make check condition for SDBBP consistent with EJTAG spec

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Date:   Tue, 9 Feb 2021 11:18:04 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-kernel@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH] MIPS: Make check condition for SDBBP consistent with
 EJTAG spec

On 02/09/2021 10:36 AM, Jiaxun Yang wrote:
>
> On Tue, Feb 9, 2021, at 12:32 AM, Jiaxun Yang wrote:
>>
>> On Mon, Feb 8, 2021, at 9:14 PM, Tiezhu Yang wrote:
>>> According to MIPS EJTAG Specification [1], a Debug Breakpoint
>>> exception occurs when an SDBBP instruction is executed, the
>>> CP0_DEBUG bit DBp indicates that a Debug Breakpoint exception
>>> occurred, just check bit DBp for SDBBP is more accurate.
>>>
>>> [1] http://www.t-es-t.hu/download/mips/md00047f.pdf
>>>
>>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>> ---
>>>   arch/mips/kernel/genex.S | 4 ++--
>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
>>> index bcce32a..6336826 100644
>>> --- a/arch/mips/kernel/genex.S
>>> +++ b/arch/mips/kernel/genex.S
>>> @@ -349,8 +349,8 @@ NESTED(ejtag_debug_handler, PT_SIZE, sp)
>>>   	MTC0	k0, CP0_DESAVE
>>>   	mfc0	k0, CP0_DEBUG
>>>   
>>> -	sll	k0, k0, 30	# Check for SDBBP.
>>> -	bgez	k0, ejtag_return
>>> +	andi	k0, k0, 0x2	# Check for SDBBP.
>>> +	beqz	k0, ejtag_return
>> You'd better define a marco for it to prevent further confusion.

OK,  thanks, I will do it in v2.

>>
>> Btw I'm curious about how do kernel receive EJTAG exception?
>> In my understanding there are only two possible EJTAG exception vectors,
>> 0xbfc00480 and DSEG one. Both of them are reachable by kernel.
> ^ not
>
>> How do this piece of code work?

We can see some useful explanations from the following comment,
the firmware needs to make sure jump to except_vec_ejtag_debug.

arch/mips/kernel/genex.S
/*
  * EJTAG debug exception handler.
  * The EJTAG debug exception entry point is 0xbfc00480, which
  * normally is in the boot PROM, so the boot PROM must do an
  * unconditional jump to this vector.
  */
NESTED(except_vec_ejtag_debug, 0, sp)
         j       ejtag_debug_handler
#ifdef CONFIG_CPU_MICROMIPS
          nop
#endif
         END(except_vec_ejtag_debug)

>>
>> Thanks.
>>
>> - Jiaxun
>>
>>>   
>>>   #ifdef CONFIG_SMP
>>>   1:	PTR_LA	k0, ejtag_debug_buffer_spinlock
>>> -- 
>>> 2.1.0
>>>
>>>
>> -- 
>> - Jiaxun
>>