linux-kernel - Re: [PATCH 1/3] arm64: dts: ti: k3-am65-main: drop RNG clock

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Message-ID: <7c63a1b5-ee51-d395-d545-6d9046d63f69@ti.com>
Date:   Fri, 4 Nov 2022 23:53:57 +0530
From:   Jayesh Choudhary <j-choudhary@...com>
To:     Nishanth Menon <nm@...com>, Andrew Davis <afd@...com>
CC:     <vigneshr@...com>, <kristo@...nel.org>, <robh+dt@...nel.org>,
        <j-keerthy@...com>, <krzysztof.kozlowski+dt@...aro.org>,
        <s-anna@...com>, <linux-arm-kernel@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] arm64: dts: ti: k3-am65-main: drop RNG clock



On 03/11/22 01:14, Nishanth Menon wrote:
> On 12:04-20221102, Andrew Davis wrote:
>> On 11/2/22 10:17 AM, Nishanth Menon wrote:
>>> On 03:02-20221101, Jayesh Choudhary wrote:
>>>> Drop RNG clock property as it is not controlled by rng-driver.
>>>
>>> Does'nt tell me what is the alternative? why is the hardware description
>>> not sufficient for control?
>>>
>>> https://software-dl.ti.com/tisci/esd/latest/5_soc_doc/am65x_sr2/clocks.html#clocks-for-sa2-ul0-device
>>> Looks like a perfectly valid description - do we have a bug and firmware
>>> does'nt allow control here?
>>>
>>
>> We have three input clocks feeding the SA2UL module, x1, x2, pka. PKA goes
>> to the PKA sub-module (isn't it nice when they make things simple). But x1 and
>> x2 are miscellaneous and bus clocks respectively and route to several sub-modules.
>>
>> All we drop here is the clock handle in the RNG sub-module, as that sub-module is
>> not the owner of that clock (the parent SA2UL is). The alternative we could implement
>> is to move the clock node up to the parent SA2UL node.
>>
>>>>
>>>> Fixes: b366b2409c97 ("arm64: dts: ti: k3-am6: Add crypto accelarator node")
>>>> Signed-off-by: Jayesh Choudhary <j-choudhary@...com>
>>>> ---
>>>>    arch/arm64/boot/dts/ti/k3-am65-main.dtsi | 1 -
>>>>    1 file changed, 1 deletion(-)
>>>>
>>>> diff --git a/arch/arm64/boot/dts/ti/k3-am65-main.dtsi b/arch/arm64/boot/dts/ti/k3-am65-main.dtsi
>>>> index 4005a73cfea9..e166d7b7e3a1 100644
>>>> --- a/arch/arm64/boot/dts/ti/k3-am65-main.dtsi
>>>> +++ b/arch/arm64/boot/dts/ti/k3-am65-main.dtsi
>>>> @@ -126,7 +126,6 @@ rng: rng@...0000 {
>>>>    			compatible = "inside-secure,safexcel-eip76";
>>>>    			reg = <0x0 0x4e10000 0x0 0x7d>;
>>>>    			interrupts = <GIC_SPI 24 IRQ_TYPE_LEVEL_HIGH>;
>>>> -			clocks = <&k3_clks 136 1>;
>>>
>>> Does this mean that the crypto module's power-domains property should be
>>> dropped as well?
>>>
>>
>> Why? the power-domains property is in the correct spot (up in the parent node).
>>
>> Now it is true we cant actually shut the SA2UL down since it is owned
>> by the security processor, but since it is marked TI_SCI_PD_SHARED this
>> should be fine.
> 
> The idea of the descriptions were to describe what is controllable by
> firmware, if there is no control due to the specified reason, it is a
> device tree bug, and should be documented when dropping it. If it serves
> a purpose in the firmware by indicating usage for example - it has valid
> reason to stick around as it is expected to be used by firmware for some
> specific reason.

The x1-clk to sa2ul is always running and fixed. What we can do is gate
off the clock specific to trng module (x1-clk) by using TRNG_EN MMR to
disable TRNG.
Hence, uncontrollable.

> 
> The commit description does bring up the above mentioned questions and
> must be explained appropriately.
> 

Okay, I will update the description properly in v2 for dropping this
clock and for each patch (and not just in the cover-letter).