linux-kernel - Re: [PATCH v6 08/12] arm64: dts: nuvoton: Add initial ma35d1 device tree

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Message-ID: <dd7940fd-bf63-552c-6e2e-05eff5fdb636@gmail.com>
Date:   Wed, 29 Mar 2023 11:43:23 +0800
From:   Jacky Huang <ychuang570808@...il.com>
To:     Stephen Boyd <sboyd@...nel.org>, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        lee@...nel.org, mturquette@...libre.com, p.zabel@...gutronix.de,
        robh+dt@...nel.org
Cc:     devicetree@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        arnd@...db.de, schung@...oton.com, mjchen@...oton.com,
        Jacky Huang <ychuang3@...oton.com>
Subject: Re: [PATCH v6 08/12] arm64: dts: nuvoton: Add initial ma35d1 device
 tree

Dear Stephen,


On 2023/3/29 上午 11:25, Stephen Boyd wrote:
> Quoting Jacky Huang (2023-03-28 20:13:11)
>> Dear Stephen,
>>
>>
>> On 2023/3/29 上午 10:46, Stephen Boyd wrote:
>>> Quoting Jacky Huang (2023-03-28 19:39:36)
>>>> On 2023/3/29 上午 10:19, Stephen Boyd wrote:
>>>>> What do you use the syscon for then? The clock driver must want to use
>>>>> the syscon for something, implying that they are the same device.
>>>> The register lock mechanism is applied to protect many critical
>>>> registers from false written.
>>>> The register lock control register is one register in system controller.
>>>> Some registers of the clock controller are lock protected. Not only
>>>> clock controller, but other
>>>> IP such as RTC, PWM, ADC, etc, also have lock protected registers. All
>>>> these IP requires
>>>> syscon to access the lock/unlock control register in the system controller.
>>>> That's why we add a <&sys> to the clock controller.
>>>>
>>>> Should we implement a ma35d1-sysctl driver to protect register_lock()
>>>> and register_unlock()
>>>> and export to those drivers?  If yes, we can remove the <&sys> from
>>>> clock controller.
>>>>
>>> You can implement the lock and unlock in the hwspinlock framework. See
>>> drivers/hwspinlock.
>> I may not explain clearly enough. The lock/unlock register of system
>> controller is more like
>> a kind of write protection for specific registers, rather than
>> preventing hetero-core CPU access.
>> In many different IP of ma35d1 contain write protected registers.
>> In fact, ma35d1 has a "hardware semaphore" IP, and we have implemented
>> the driver in drivers/hwspinlock.
>> Even the control register of "hardware semaphore" is also write protected.
> What's the need to lock and unlock the registers? Is some other
> processor also writing to the registers that we need to synchronize
> against? Or is Linux the only entity reading and writing the registers?
> I'm wondering if we should simply unlock the registers and never lock
> them.
>
>> So, should we implement a system controller driver to provide
>> register_unlock() function?
>> Is it OK to have such a driver in drivers/mfd?
>> Or, just use syscon in device tree for those devices that have write
>> protect registers.
>>
> The hwspinlock framework doesn't require there to be another entity
> accessing some resource. It's there to implement hardware locks. I don't
> see why it can't be used here.

The current usage of register lock/unlock protect is as the following code:

static void ma35d1_unlock_regs(struct ma35d1_clk_pll *pll)
{
     int ret;

     do {
         regmap_write(pll->regmap, REG_SYS_RLKTZNS, 0x59);
         regmap_write(pll->regmap, REG_SYS_RLKTZNS, 0x16);
         regmap_write(pll->regmap, REG_SYS_RLKTZNS, 0x88);
         regmap_read(pll->regmap, REG_SYS_RLKTZNS, &ret);
     } while (ret == 0);
}

static void ma35d1_lock_regs(struct ma35d1_clk_pll *pll)
{
     regmap_write(pll->regmap, REG_SYS_RLKTZNS, 0x0);
}

And the following code is to unlock registers for write and then lock again.

     ma35d1_unlock_regs(pll);
     writel_relaxed(reg_ctl[0], pll->ctl0_base);
     writel_relaxed(reg_ctl[1], pll->ctl1_base);
     writel_relaxed(reg_ctl[2], pll->ctl2_base);
     ma35d1_lock_regs(pll);

The above code is from the clk-ma35d1-pll.c from this patchset.

We just employ regmap mechansim for the access to REG_SYS_RLKTZNS register.
Is this implementation OK for you?  Thank you.


Best regards,
Jacky Huang