linux-kernel - Re: [PATCH v1 2/3] soc: mediatek: mtk-cmdq: Mark very unlikely branches as such

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Message-ID: <06c5573b-8b63-4a75-8af2-d6aace86fe69@collabora.com>
Date: Wed, 2 Oct 2024 14:43:47 +0200
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
To: Matthias Brugger <matthias.bgg@...il.com>,
 linux-mediatek@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 kernel@...labora.com
Subject: Re: [PATCH v1 2/3] soc: mediatek: mtk-cmdq: Mark very unlikely
 branches as such

Il 02/10/24 14:41, Matthias Brugger ha scritto:
> 
> 
> On 18/09/2024 12:06, AngeloGioacchino Del Regno wrote:
>> Calling cmdq packet builders with an unsupported event number,
>> or without left/right operands (in the case of logic commands)
>> means that the caller (another driver) wants to perform an
>> unsupported operation, so this means that the caller must be
>> fixed instead.
>>
>> Anyway, such checks are here for safety and, unless any driver
>> bug or any kind of misconfiguration is present, will always be
>> false so add a very unlikely hint for those.
>>
>> Knowing that CPUs' branch predictors (and compilers, anyway) are
>> indeed smart about these cases, this is done mainly for human
>> readability purposes.
>>
> 
> Are you really sure we need that? As you mentioned the unlikely() is probably 
> useless as compiler and branch predictions will do the job. I don't see the 
> complexity in the code to have this annotations for human readability.
> 
> I would argue against using unlikely() here as, in general, it is discouraged to 
> use it. We will just create a data point of doing things that should only be done 
> with very good reason. I don't see the reason here, it will only confuse other 
> developers about the use of likely() and unlikely().
> 

If you have strong opinions I have no problem dropping this.
Perhaps I can add a comment stating "this is very unlikely to happen and should
be dropped after thorough cleanup", if that's better?

Cheers!
Angelo

> Regards,
> Matthias
> 
>> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
>> ---
>>   drivers/soc/mediatek/mtk-cmdq-helper.c | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/soc/mediatek/mtk-cmdq-helper.c 
>> b/drivers/soc/mediatek/mtk-cmdq-helper.c
>> index 620c371fd1fc..4ffd1a35df87 100644
>> --- a/drivers/soc/mediatek/mtk-cmdq-helper.c
>> +++ b/drivers/soc/mediatek/mtk-cmdq-helper.c
>> @@ -336,7 +336,7 @@ int cmdq_pkt_wfe(struct cmdq_pkt *pkt, u16 event, bool clear)
>>       struct cmdq_instruction inst = { {0} };
>>       u32 clear_option = clear ? CMDQ_WFE_UPDATE : 0;
>> -    if (event >= CMDQ_MAX_EVENT)
>> +    if (unlikely(event >= CMDQ_MAX_EVENT))
>>           return -EINVAL;
>>       inst.op = CMDQ_CODE_WFE;
>> @@ -351,7 +351,7 @@ int cmdq_pkt_acquire_event(struct cmdq_pkt *pkt, u16 event)
>>   {
>>       struct cmdq_instruction inst = {};
>> -    if (event >= CMDQ_MAX_EVENT)
>> +    if (unlikely(event >= CMDQ_MAX_EVENT))
>>           return -EINVAL;
>>       inst.op = CMDQ_CODE_WFE;
>> @@ -366,7 +366,7 @@ int cmdq_pkt_clear_event(struct cmdq_pkt *pkt, u16 event)
>>   {
>>       struct cmdq_instruction inst = { {0} };
>> -    if (event >= CMDQ_MAX_EVENT)
>> +    if (unlikely(event >= CMDQ_MAX_EVENT))
>>           return -EINVAL;
>>       inst.op = CMDQ_CODE_WFE;
>> @@ -381,7 +381,7 @@ int cmdq_pkt_set_event(struct cmdq_pkt *pkt, u16 event)
>>   {
>>       struct cmdq_instruction inst = {};
>> -    if (event >= CMDQ_MAX_EVENT)
>> +    if (unlikely(event >= CMDQ_MAX_EVENT))
>>           return -EINVAL;
>>       inst.op = CMDQ_CODE_WFE;
>> @@ -476,7 +476,7 @@ int cmdq_pkt_logic_command(struct cmdq_pkt *pkt, u16 
>> result_reg_idx,
>>   {
>>       struct cmdq_instruction inst = { {0} };
>> -    if (!left_operand || !right_operand || s_op >= CMDQ_LOGIC_MAX)
>> +    if (unlikely(!left_operand || !right_operand || s_op >= CMDQ_LOGIC_MAX))
>>           return -EINVAL;
>>       inst.op = CMDQ_CODE_LOGIC;

-- 
AngeloGioacchino Del Regno
Senior Software Engineer

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