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

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Message-ID: <5c5e3f7d-5ade-4625-bef9-4fcb7f717778@collabora.com>
Date: Wed, 2 Oct 2024 15:02:51 +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 14:58, Matthias Brugger ha scritto:
> 
> 
> 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.
> 

Let's drop it then. :-)

>> 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)?
> 

The reasoning behind all of this is that those functions are used in drivers'
performance paths, including display and camera.... so a WARN_ON would be even
more against what I'm trying to do.

At this point we can just leave this as it is....

> 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;
>>