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Date:   Thu, 18 May 2023 09:38:41 +0100
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     "lihuisong (C)" <lihuisong@huawei.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Bjorn Andersson <andersson@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        Shawn Guo <shawnguo@kernel.org>, linux-kernel@vger.kernel.org,
        soc@kernel.org, wanghuiqiang@huawei.com, tanxiaofei@huawei.com,
        liuyonglong@huawei.com, huangdaode@huawei.com,
        linux-acpi@vger.kernel.org, Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        devicetree@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Subject: Re: [PATCH] soc: hisilicon: Support HCCS driver on Kunpeng SoC
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On Thu, May 18, 2023 at 04:24:36PM +0800, lihuisong (C) wrote:
> 
> 在 2023/5/17 21:16, Sudeep Holla 写道:
> > On Wed, May 17, 2023 at 07:35:25PM +0800, lihuisong (C) wrote:
> > > 在 2023/5/17 17:30, Sudeep Holla 写道:
> > > > On Wed, May 17, 2023 at 03:16:12PM +0800, lihuisong (C) wrote:
> > > > 
> > > > [...]
> > > > 
> > > > > No. I want to use this flag to make compability between different platforms.
> > > > > This driver only use PCC OpRegion to access to the channel if platform
> > > > > support use PCC OpRegion.
> > > > What do you mean by that ? It is not correct. If there is a PCC Opregion,
> > > > then you need to make it work with drivers/acpi/acpi_pcc.c
> > > > 
> > > > You need to have all the other details in the firmware(ASL). By looking
> > > > at the driver, it has no connection to PCC Opregion IMO unless I am missing
> > > > something.
> > > Driver just needs to call these APIs, such as acpi_evaluate_integer(), if
> > > want to use PCC OpRegion.
> > OK, please provide examples. I am definitely lost as it doesn't match with
> > my understanding of how PCC Opregions are/can be used.
> > 
> > > I know that. I have tested PCC OpRegion before.
> > Cool, examples please.
> > 
> > > You've completely misunderstood what I said.????
> > > 
> > Hmm, may be but I need examples.
> As you said below, the driver works just for PCC not PCC Opregion for now.
> not sure if we need to discuss how PCC Opregion is used here.

Good let us drop the idea of using PCC Opregion with this driver for now.

> > 
> > > I mean that this driver plans to support both PCC and PCC OpRegion.
> > > For example,
> > > Platform A: this driver use PCC (as the current implementation)
> > Good, then just keep what it needs in the implementation nothing more
> > until you add support for something you have described below(not that
> > I agree, just want you to make progress here based on what is actually
> > required today)
> Agreed.
> > 
> > > Platform B: this driver use PCC OpRegion (Currently, this patch does not
> > > implement it, but it may be available in the future.)
> > Then let us discuss that in the future, don't add unnecessary complexity
> > for some future use case today. You can always add it when you introduce
> > that feature or support in the future.
> Yes. We just need to focus on the current.
> If there are any usage problems with PCC OpRegion in the future, we can
> discuss that later.
>

Agreed.

> My original full scheme is as follows:
> -->
> dev_flags = get_device_flags();  // to know if use PCC OpRegion
> if (USE_PCC_OPREGION_B in dev_flags is 0) {
>     chan_id = get_pcc_chan_id();
>     init_mbox_client();
>     pcc_mbox_request_channel(cl, chan_id)
> } else {
>     /* we need to return unsupport now because of no this feature in this
> driver. */
>     do_nothing();
> }
> 
> void get_some_info(...) {
>     if (USE_PCC_OPREGION_B in dev_flags is 0)
>         pcc_cmd_send();  // use PCC to communicate with Platform
>     else
>         acpi_evaluate_object(); // will be used in future.
> }
> 
> As described in the pseudocode above,
> it is necessary to put "dev_flags" in this current driver first in case of
> the version driver runs on the platform which just use PCC Opregion.

No, you can't randomly define dev_flags just to assist your driver
implementation. If you need it, you need to get the spec updated. We
will not add anything unless that happens.

Note that I don't agree with the flags at all but if you convince and get
them added to spec, I won't object.

> >
> > > Note:
> > > This driver selects only one of them (PCC and PCC OpRegion) to communicate
> > > with firmware on one platform.
> > Let us keep it simple(KISS). The driver works just for PCC not PCC Opregion
> > for now.
> ok.

Good

> > 
> > > We use one bit in device-flags to know which one this driver will use.
> > > 
> > NACK again just to re-iterate my point if you have not yet accepted that
> > fact.
> Above is our plan. Do you still think we shouldn't add this device-flags?
> please let me know.

Correct, no device flags as I see no use for it with your PCC only use case
for now, right ?

> > > I'm not sure if you can understand what I mean by saing that.
> > > If you're not confused about this now, can you reply to my last email
> > > again?????
> > > 
> > The example you had IIRC is use of System Memory Opregion to demonstrate
> > some _DSM. That has nothing to do with PCC Opregion.
> Yes, it doesn't matter.
> I just want to have a way to get device-flags which contains many bits(every
> bits can be used to as one feature for expanding), rigtht?

Get it through the spec, we don't allow random additions for some
implementations like this.

> > 
> > Commit 77e2a04745ff ("ACPI: PCC: Implement OperationRegion handler for
> > the PCC Type 3 subtype") has the example in the commit message. IIRC,
> Your example is very useful to the user.
> > you have even fixed couple of bugs in that driver. That is the reason
> > why I don't understand how you think this driver and that can or must
> Understand you, Sudeep.
> At that time, I tested it by a simple demo driver on the platform supported
> type3.
>

OK

> This driver will support multiple platforms.
> On some platforms, we can only use PCC with polling way.
> And we will add PCC Opregion way for others platforms.

Again when you do please post the patch with the ASL snippet as I am
very much interested in understanding how you would make that work.

> What's more, every platform just use one of them(PCC and PCC Opregion).

OK

> > work together. At least I fail to see how ATM(examples please, by that
> > I mean ASL snippet for PCC vs PCC Opregion usage to work with this driver)
> ok!
> For PCC, ASL snippet is little.
> I will add ASL snippet when this driver addes PCC Opregion way.


Sounds like a plan to make progress at-least for now.

-- 
Regards,
Sudeep