linux-kernel - Re: [PATCH v2 3/6] clk: sunxi-ng: nkm: Support minimum and maximum rate

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Message-ID: <874je6b2g4.fsf@oltmanns.dev>
Date: Sun, 18 Feb 2024 09:29:15 +0100
From: Frank Oltmanns <frank@...manns.dev>
To: Maxime Ripard <mripard@...nel.org>
Cc: Michael Turquette <mturquette@...libre.com>, Stephen Boyd
 <sboyd@...nel.org>, Chen-Yu Tsai <wens@...e.org>, Jernej Skrabec
 <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, Guido
 Günther <agx@...xcpu.org>, Purism Kernel Team
 <kernel@...i.sm>, Ondrej
 Jirman <megi@....cz>, Neil Armstrong <neil.armstrong@...aro.org>, Jessica
 Zhang <quic_jesszhan@...cinc.com>, Sam Ravnborg <sam@...nborg.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann
 <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter
 <daniel@...ll.ch>, Rob Herring <robh+dt@...nel.org>, Krzysztof Kozlowski
 <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>,
 linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 3/6] clk: sunxi-ng: nkm: Support minimum and maximum
 rate

Hi Maxime,

On 2024-02-08 at 13:16:27 +0100, Maxime Ripard <mripard@...nel.org> wrote:
> [[PGP Signed Part:Undecided]]
> On Mon, Feb 05, 2024 at 04:22:26PM +0100, Frank Oltmanns wrote:
>> According to the Allwinner User Manual, the Allwinner A64 requires
>> PLL-MIPI to run at 500MHz-1.4GHz. Add support for that to ccu_nkm.
>>
>> Signed-off-by: Frank Oltmanns <frank@...manns.dev>
>> ---
>>  drivers/clk/sunxi-ng/ccu_nkm.c | 13 +++++++++++++
>>  drivers/clk/sunxi-ng/ccu_nkm.h |  2 ++
>>  2 files changed, 15 insertions(+)
>>
>> diff --git a/drivers/clk/sunxi-ng/ccu_nkm.c b/drivers/clk/sunxi-ng/ccu_nkm.c
>> index 1168d894d636..7d135908d6e0 100644
>> --- a/drivers/clk/sunxi-ng/ccu_nkm.c
>> +++ b/drivers/clk/sunxi-ng/ccu_nkm.c
>> @@ -181,6 +181,12 @@ static unsigned long ccu_nkm_round_rate(struct ccu_mux_internal *mux,
>>  	if (nkm->common.features & CCU_FEATURE_FIXED_POSTDIV)
>>  		rate *= nkm->fixed_post_div;
>>
>> +	if (nkm->min_rate && rate < nkm->min_rate)
>> +		rate = nkm->min_rate;
>> +
>> +	if (nkm->max_rate && rate > nkm->max_rate)
>> +		rate = nkm->max_rate;
>> +
>
> This is provided by the clock range already. If you call
> clk_hw_set_rate_range, it should work just fine.

I have to admit, that I don't know that much about sunxi-ng or the CCF
and therefore humbly request some guidance.

I've looked at other examples of clk_hw_set_rate_range() usage and it
seems there is not a lot of adoption for this functionality even though
it was already introduced mid-2015. This makes me wonder, why that is.

Anyhow, it seems in all examples I found, clk_hw_set_rate_range() is
called immediately after registering the clk_hw. So, in the case of
sunxi-ng, we'd need to do that in sunxi_ccu_probe, which is a common
function for all sunxi-ng clock types. Correct?

If so, surely, you don't want me to introduce clock type specific code
to a common function, so I assume you want min_rate and max_rate to
become members of struct ccu_common. Correct?

If so, since there already are some clock types in sunxi-ng that support
having a minimum and maximum rate, these clocks should be refactored
eventually. Correct?

Finally, in sunxi-ng there is a feature of having a fixed_post_div (see,
e.g., the first to lines of the diff above). It seems to me that CCF
cannot know about these post_divs, so we'd also need to transfer the
fixed_post_div to ccu_common and use that when calling
clk_hw_set_rate_range. Correct?

The fact that you casually dropped the two sentences above and me
deducing you want a somewhat large refactoring of the functionality for
sunxi-ng, makes me wonder if I completely misunderstood your request.

Best regards,
  Frank

>
> Maxime
>
> [[End of PGP Signed Part]]