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

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Message-ID: <2cb6e2d6-226f-4a40-bdff-81baff8988a6@gmail.com>
Date: Wed, 2 Oct 2024 14:58:26 +0200
From: Matthias Brugger <matthias.bgg@...il.com>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.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



On 02/10/2024 14:43, AngeloGioacchino Del Regno wrote:
> 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.

My take would be to drop it.

> Perhaps I can add a comment stating "this is very unlikely to happen and should
> be dropped after thorough cleanup", if that's better?
> 

As these are exported functions they could be used by out-of-tree modules, so it 
could make sense to check the input parameter. Maybe transform it to 
WARN_ON(event >= CMDQ_MAX_EVENT)?

Regards,
Matthias

> 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;
>