linux-kernel - Re: [PATCH v11 1/4] firmware: qcom: scm: provide a read-modify-write function

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Date: Tue, 20 Feb 2024 15:41:26 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Bjorn Andersson <andersson@...nel.org>,
        Linus Walleij
	<linus.walleij@...aro.org>
CC: Mark Brown <broonie@...nel.org>, <konrad.dybcio@...aro.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v11 1/4] firmware: qcom: scm: provide a read-modify-write
 function



On 2/17/2024 12:01 AM, Bjorn Andersson wrote:
> On Tue, Jan 09, 2024 at 02:34:10PM +0100, Linus Walleij wrote:
>> On Tue, Jan 9, 2024 at 2:24 PM Mukesh Ojha <quic_mojha@...cinc.com> wrote:
>>> On 1/9/2024 6:44 PM, Linus Walleij wrote:
>>>> On Mon, Jan 8, 2024 at 4:28 PM Mukesh Ojha <quic_mojha@...cinc.com> wrote:
>>>>
>>>>> It was realized by Srinivas K. that there is a need of
>>>>> read-modify-write scm exported function so that it can
>>>>> be used by multiple clients.
>>>>>
>>>>> Let's introduce qcom_scm_io_rmw() which masks out the bits
>>>>> and write the passed value to that bit-offset.
>>>> (...)
>>>>> +int qcom_scm_io_rmw(phys_addr_t addr, unsigned int mask, unsigned int val)
>>>>> +{
>>>>> +       unsigned int old, new;
>>>>> +       int ret;
>>>>> +
>>>>> +       if (!__scm)
>>>>> +               return -EINVAL;
>>>>> +
>>>>> +       spin_lock(&__scm->lock);
>>>>> +       ret = qcom_scm_io_readl(addr, &old);
>>>>> +       if (ret)
>>>>> +               goto unlock;
>>>>> +
>>>>> +       new = (old & ~mask) | (val & mask);
>>>>> +
>>>>> +       ret = qcom_scm_io_writel(addr, new);
>>>>> +unlock:
>>>>> +       spin_unlock(&__scm->lock);
>>>>> +       return ret;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(qcom_scm_io_rmw);
>>>>
>>>> This looks a lot like you are starting to re-invent regmaps
>>>> regmap_update_bits().
>>>>
>>>> If you are starting to realize you need more and more of
>>>> regmap, why not use regmap and its functions?
>>>
>>> I think, this discussion has happened already ..
>>>
>>> https://lore.kernel.org/lkml/CACRpkdb95V5GC81w8fiuLfx_V1DtWYpO33FOfMnArpJeC9SDQA@mail.gmail.com/
>>
>> That discussion ended with:
>>
>> [Bjorn]
>>> We'd still need qcom_scm_io_readl() and qcom_scm_io_writel() exported to
>>> implement the new custom regmap implementation - and the struct
>>> regmap_config needed in just pinctrl-msm alone would be larger than the
>>> one function it replaces.
>>
>> When you add more and more accessors the premise starts to
>> change, and it becomes more and more of a reimplementation.
>>
>> It may be time to actually fix this.
>>
> 
> Thought I had replied to this already, did we discuss this previously as
> well?
> 
> My concern with expressing this as a regmap is that from the provider's
> point of view, the regmap would span the entire 32-bit address space.
> I'm guessing that there's something on the other side limiting what
> subregions are actually accessible for each platform/firmware
> configuration, but I'm not convinced that regmap a good abstraction...

To add more to it, in current series, we are just accessing single 
register for read/write and using regmap for this looks overkill to
me.

-Mukesh
> 
> Regards,
> Bjorn
> 
>> Yours,
>> Linus Walleij