Re: [PATCH v4 07/11] ASoC: jz4740-i2s: Make the PLL clock name SoC-specific

[Aidan MacDonald]

Paul Cercueil <paul@xxxxxxxxxxxxxxx> writes:

> Hi Aidan,
>
> Le dim. 23 oct. 2022 à 14:29:24 +0100, Aidan MacDonald
> <aidanmacdonald.0x0@xxxxxxxxx> a écrit :
>> Paul Cercueil <paul@xxxxxxxxxxxxxxx> writes:
>>
>>>  Hi Aidan,
>>>  Le sam. 22 oct. 2022 à 18:15:05 +0100, Aidan MacDonald
>>>  <aidanmacdonald.0x0@xxxxxxxxx> a écrit :
>>>>  Actually, the clock names in the DT are meaningless. The clk_get() call
>>>>  matches only the clock's name in the CGU driver. So in fact the driver
>>>>  is "broken" for jz4780. It seems jz4770 doesn't work correctly either,
>>>>  it has no "pll half", and three possible parents for its "i2s" clock.
>>>  That's not true. The clock names are matched via DT.
>>>  Only in the case where a corresponding clock cannot be found via DT will it
>>>  search for the clock name among the clock providers. I believe this is a
>>> legacy
>>>  mechanism and you absolutely shouldn't rely on it.
>>>  -Paul
>>>
>> What you say is only true for clk_get() with a device argument. When the
>> device argument is NULL -- which is the case in .set_sysclk() -- then
>> the DT name is not matched. Check drivers/clk/clkdev.c, in clk_find().
>> When the dev_id is NULL, it will not match any lookup entries with a
>> non-null dev_id, and I believe dev_id is the mechanism that implements
>> DT clock lookup. Only the wildcard entries from the CGU driver will be
>> matched if dev_id is NULL, so the DT is being ignored.
>> If you don't believe me, try changing "pll half" in the device tree and
>> the I2S driver to something else. I have done this, and it doesn't work.
>> That proves the name in the device tree is not being used.
>
> Well, let's pass them a device pointer then.
>

Yes, I'll do that when I revise the patch.

>> I agree we shouldn't rely on this, it's a legacy behavior, but the fact
>> is that's how the driver already works. I'm dropping this patch because
>> the driver is wrong and needs a different fix...
>
> "How the driver already works" is a bit misleading, I never saw this
> .set_sysclk() callback being called, so I can't really say that it works.
>
>>>>  I think a better approach is to have the DT define an array of parent
>>>>  clocks for .set_sysclk()'s use, instead of hardcoding parents in the
>>>>  driver. If the parent array is missing the driver can default to using
>>>>  "ext" so existing DTs will work.
>> As much as I like this idea there doesn't seem to be a mechanism for
>> handling a free-floating array of clocks in the DT. Everything has
>> to be put in the main "clocks" array. That makes it pretty hard to
>> figure out which ones are meant to be the parent clocks.
>> Do you know of any way to do this generically from the DT? If there's
>> no way to get away from a hardcoded array of names in the driver, I can
>> at least add a device argument to clk_get() so it'll use the DT names.
>
> In jz4740_i2s_set_sysclk():
>
> #define JZ4740_I2S_FIRST_PARENT_CLK 2
> parent = of_clk_get(dev->of_node, JZ4740_I2S_FIRST_PARENT_CLK + clk_id);
>
> is how I'd do it.
>
> The DTs all have "aic", "i2s" as the first two clocks. It is even enforced in
> the DT schemas.
>
> Cheers,
> -Paul

Sounds like a plan. I was hoping to avoid adding CONFIG_OF back
considering I removed it in an earlier patch since it was unused. :)

Guess it doesn't really matter for this driver since the Ingenic
SoCs need CONFIG_OF anyway.

Regards,
Aidan