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Date: Wed, 5 Mar 2025 19:08:07 +0100
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
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Subject: Re: [PATCH v4 6/8] soc: mediatek: Add programming flow for
 unsupported subsys ID hardware

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@...iatek.com>
>>> ---
>>>    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@mediatek.com/
> 
> 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

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