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

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Message-ID: <7d577541-cada-4f6f-8e83-c420dd0e9382@gmail.com>
Date: Wed, 2 Oct 2024 14:41:22 +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 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().

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;