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

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Date:   Mon, 24 Oct 2022 14:06:13 +0100
From:   Paul Cercueil <paul@...pouillou.net>
To:     Aidan MacDonald <aidanmacdonald.0x0@...il.com>
Cc:     Zhou Yanjie <zhouyu@...yeetech.com>, lgirdwood@...il.com,
        broonie@...nel.org, perex@...ex.cz, tiwai@...e.com,
        linux-mips@...r.kernel.org, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 07/11] ASoC: jz4740-i2s: Make the PLL clock name
 SoC-specific

Hi Aidan,

Le dim. 23 oct. 2022 à 14:29:24 +0100, Aidan MacDonald 
<aidanmacdonald.0x0@...il.com> a écrit :
> 
> Paul Cercueil <paul@...pouillou.net> writes:
> 
>>  Hi Aidan,
>> 
>>  Le sam. 22 oct. 2022 à 18:15:05 +0100, Aidan MacDonald
>>  <aidanmacdonald.0x0@...il.com> a écrit :
>>>  Zhou Yanjie <zhouyu@...yeetech.com> writes:
>>> 
>>>>   Hi Paul,
>>>>   On 2022/7/13 下午11:07, Paul Cercueil wrote:
>>>>>   Hi Zhou,
>>>>>   Le mer., juil. 13 2022 at 22:33:44 +0800, Zhou Yanjie
>>>>>  <zhouyu@...yeetech.com>
>>>>>   a écrit :
>>>>>>   Hi Aidan,
>>>>>>   On 2022/7/9 上午12:02, Aidan MacDonald wrote:
>>>>>>>   @@ -400,6 +402,7 @@ static const struct i2s_soc_info 
>>>>>>> jz4740_i2s_soc_info
>>>>>>>  =
>>>>>>>   {
>>>>>>>         .field_tx_fifo_thresh    = REG_FIELD(JZ_REG_AIC_CONF, 
>>>>>>> 8, 11),
>>>>>>>         .field_i2sdiv_capture    = 
>>>>>>> REG_FIELD(JZ_REG_AIC_CLK_DIV, 0, 3),
>>>>>>>         .field_i2sdiv_playback    = 
>>>>>>> REG_FIELD(JZ_REG_AIC_CLK_DIV, 0, 3),
>>>>>>>   +    .pll_clk_name        = "pll half",
>>>>>>>         .shared_fifo_flush    = true,
>>>>>>>     };
>>>>>>   Since JZ4760, according to the description of the I2SCDR 
>>>>>> register,
>>>>>>   Ingenic SoCs no longer use PLL/2 clock, but directly use PLL 
>>>>>> clock,
>>>>>>   so it seems also inappropriate to use "pll half" for these 
>>>>>> SoCs.
>>>>>   The device tree passes the clock as "pll half". So the driver 
>>>>> should use
>>>>>  this
>>>>>   name as well...
>>>>   I see...
>>>>   It seems that the device tree of JZ4770 has used "pll half" 
>>>> already,
>>>>   but there is no "pll half" used anywhere in the device tree of 
>>>> JZ4780,
>>>>   maybe we can keep the pll_clk_name of JZ4770 as "pll half", and 
>>>> change
>>>>   the pll_clk_name of JZ4780 to a more reasonable name.
>>>>   Thanks and best regards!
>>>  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.

> 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