linux-kernel - Re: [PATCH v2] MIPS: Make check condition for SDBBP consistent with EJTAG spec

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Date:   Tue, 9 Feb 2021 21:09:52 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH v2] MIPS: Make check condition for SDBBP consistent with
 EJTAG spec

On 02/09/2021 08:11 PM, Thomas Bogendoerfer wrote:
> On Tue, Feb 09, 2021 at 01:05:25PM +0800, 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>
>> ---
>>
>> v2: add MIPS_DEBUG_DBP definition
>>
>>   arch/mips/include/asm/mipsregs.h | 4 ++++
>>   arch/mips/kernel/genex.S         | 4 ++--
>>   2 files changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
>> index a0e8ae5..9c8099a 100644
>> --- a/arch/mips/include/asm/mipsregs.h
>> +++ b/arch/mips/include/asm/mipsregs.h
>> @@ -1085,6 +1085,10 @@
>>   #define CVMVMCONF_RMMUSIZEM1_S	0
>>   #define CVMVMCONF_RMMUSIZEM1	(_U64CAST_(0xff) << CVMVMCONF_RMMUSIZEM1_S)
>>   
>> +/* Debug register field definitions */
>> +#define MIPS_DEBUG_DBP_SHIFT	1
>> +#define MIPS_DEBUG_DBP		(_ULCAST_(1) << MIPS_DEBUG_DBP_SHIFT)
>> +
>>   /*
>>    * Coprocessor 1 (FPU) register names
>>    */
>> diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
>> index bcce32a..743d759 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, MIPS_DEBUG_DBP	# Check for SDBBP.
>> +	beqz	k0, ejtag_return
> IMHO both implementations are doing the same thing.

When I read the original code, it looks a little confusing
at first glance, the initial aim of this patch is to make the code
more readable and easier to understand.

>
> Thomas.
>