Re: [PATCH v4 6/8] soc: mediatek: Add programming flow for unsupported subsys ID hardware

[AngeloGioacchino Del Regno]

Il 05/03/25 17:12, Jason-JH Lin (林睿祥) ha scritto:
> On Tue, 2025-03-04 at 10:41 +0100, AngeloGioacchino Del Regno wrote:
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.
> >
> >
> > Il 18/02/25 06:41, Jason-JH Lin ha scritto:
> > > To support hardware without subsys IDs on new SoCs, add a
> > > programming
> > > flow that checks whether the subsys ID is valid. If the subsys ID
> > > is
> > > invalid, the flow will call 2 alternative CMDQ APIs:
> > > cmdq_pkt_assign() and cmdq_pkt_write_s_value() to achieve the same
> > > functionality.
> > >
> > > Signed-off-by: Jason-JH Lin <jason-jh.lin@xxxxxxxxxxxx>
> > > ---
> > >    drivers/soc/mediatek/mtk-mmsys.c | 14 +++++++++++---
> > >    drivers/soc/mediatek/mtk-mutex.c | 11 +++++++++--
> > >    2 files changed, 20 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/soc/mediatek/mtk-mmsys.c
> > > b/drivers/soc/mediatek/mtk-mmsys.c
> > > index bb4639ca0b8c..ce949b863b05 100644
> > > --- a/drivers/soc/mediatek/mtk-mmsys.c
> > > +++ b/drivers/soc/mediatek/mtk-mmsys.c
> > > @@ -167,9 +167,17 @@ static void mtk_mmsys_update_bits(struct
> > > mtk_mmsys *mmsys, u32 offset, u32 mask,
> > >        u32 tmp;
> > >
> > >        if (mmsys->cmdq_base.size && cmdq_pkt) {
> > > -             ret = cmdq_pkt_write_mask(cmdq_pkt, mmsys-
> > > > cmdq_base.subsys,
> > > -                                       mmsys->cmdq_base.offset +
> > > offset, val,
> > > -                                       mask);
> > > +             offset += mmsys->cmdq_base.offset;
> > > +             if (mmsys->cmdq_base.subsys != CMDQ_SUBSYS_INVALID) {
> >
> > You're still anyway passing the subsys to cmdq_pkt_write_mask(),
> > right?!
> > Why don't you just handle this in cmdq_pkt_write_mask() then? ;-)
> >
> > I can see this pattern being repeated over and over in both
> > drm/mediatek and MDP3
> > drivers, and it's not necessary to duplicate this many times when you
> > can write it
> > just once.
> >
> > Would've also been faster for you to implement... :-D
>
> I think did it in the series V1:
> https://patchwork.kernel.org/project/linux-mediatek/patch/20241121042602.32730-5-jason-jh.lin@xxxxxxxxxxxx/
>
> Because it'll need to passing the base_pa and that will need to change
> the interface for original APIs.
>
> And CK think that's not a necessary to change the APIs. It can be done
> by cmdq_pkt_assign() + cmdq_pkt_write_s_mask_value() in the client
> drivers. Then you can see this pattern in everywhere. :-)

Using likely(x) and unlikely(x) should be avoided, really, unless it's something
that is really really really really ... really ... rea.... likely or unlikely :-)

Btw. Changing the APIs is a bit difficult, but I disagree with CK about not
"inventing" a new API for the unsupported-subsys flow.

It's true, it is not *strictly* needed to add a function, but it's good for any
kind of future maintainability - as I explained, it's easier then to fix a problem
if there's one.... and well, I can see that you agree with me, because effectively
you did it the first time :-)

CK mentioned using cmdq_pkt_write() *or* cmdq_pkt_assignwrite/cmdq_pkt_write_pa()
(however you wanna call it, it's fine for me), in drivers that know that there
always is or there always isn't a subsys ID: that's a good suggestion, as this can
be eventually done with assigning a function pointer, so, no conditionals at each
operation.

My point of view, finally, is:
 - This is just another way of doing cmdq_pkt_write()
   - This, at the end of the day, does exactly what cmdq_pkt_write() is doing,
     except it's doing it with two instructions instead of one;
 - The same thing can be done in two different ways (depending on SoC)
   - This same thing should have a function that does it.

A function that does it can be

int cmdq_pkt_write_pa(struct cmdq_pkt *pkt, u8 subsys /*unused*/, u32 pa_base, u16 
offset, u32 value)
{
	err = cmdq_pkt_assign(pkt, 0, CMDQ_ADDR_HIGH(pa_base));
	if (err < 0)
		return err;

	return cmdq_pkt_write_s_value( .... etc)
}

int cmdq_pkt_write() <--- unchanged, scheduled for removal after all drivers migrated

int cmdq_pkt_write_subsys(struct cmdq_pkt *pkt, u8 subsys, u32 pa_base /*unused*/, 
u16 offset, u32 value)
{
	/* This function will get the contents of cmdq_pkt_write once removed,
           but, in the meanwhile, to avoid duplication we just call that: */

	return cmdq_pkt_write(pkt, subsys, offset, value);
}

- Are we adding one more function parameter? Yes
- Is this impacting performance overall? Not really

After all, we're living in an ARMv8 (actually, ARMv9 for new ones) world, so
one more function param won't hurt anyone.

I think that's the best of both worlds, and makes everyone happy.
Are you happy with that? :-)

Cheers,
Angelo