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

[AngeloGioacchino Del Regno]

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@xxxxxxxxxxxxx>
> > > > ---
> > > >   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;