linux-kernel - Re: [PATCH] firmware: imx: Skip return value check for some special SCU firmware APIs

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Date:   Mon, 30 Sep 2019 07:28:54 +0000
From:   Leonard Crestez <leonard.crestez@....com>
To:     Anson Huang <anson.huang@....com>,
        Marco Felsch <m.felsch@...gutronix.de>,
        Aisheng Dong <aisheng.dong@....com>
CC:     "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH] firmware: imx: Skip return value check for some special
 SCU firmware APIs

On 2019-09-27 4:20 AM, Anson Huang wrote:
>> On 2019-09-26 1:06 PM, Marco Felsch wrote:
>>> On 19-09-26 08:03, Anson Huang wrote:
>>>>> On 19-09-25 18:07, Anson Huang wrote:
>>>>>> The SCU firmware does NOT always have return value stored in
>>>>>> message header's function element even the API has response data,
>>>>>> those special APIs are defined as void function in SCU firmware, so
>>>>>> they should be treated as return success always.
>>>>>>
>>>>>> +static const struct imx_sc_rpc_msg whitelist[] = {
>>>>>> +	{ .svc = IMX_SC_RPC_SVC_MISC, .func =
>>>>> IMX_SC_MISC_FUNC_UNIQUE_ID },
>>>>>> +	{ .svc = IMX_SC_RPC_SVC_MISC, .func =
>>>>>> +IMX_SC_MISC_FUNC_GET_BUTTON_STATUS }, };
>>>>>
>>>>> Is this going to be extended in the near future? I see some upcoming
>>>>> problems here if someone uses a different scu-fw<->kernel
>>>>> combination as nxp would suggest.
>>>>
>>>> Could be, but I checked the current APIs, ONLY these 2 will be used
>>>> in Linux kernel, so I ONLY add these 2 APIs for now.
>>>
>>> Okay.
>>>
>>>> However, after rethink, maybe we should add another imx_sc_rpc API
>>>> for those special APIs? To avoid checking it for all the APIs called which
>> may impact some performance.
>>>> Still under discussion, if you have better idea, please advise, thanks!
>>
>> My suggestion is to refactor the code and add a new API for the this "no
>> error value" convention. Internally they can call a common function with
>> flags.
> 
> If I understand your point correctly, that means the loop check of whether the API
> is with "no error value" for every API still NOT be skipped, it is just refactoring the code,
> right?

There would be no "loop" anywhere: the responsibility would fall on the 
call to call the right RPC function. In the current layering scheme 
(drivers -> RPC -> mailbox) the RPC layer treats all calls the same and 
it's up the the caller to provide information about calling convention.

An example implementation:
* Rename imx_sc_rpc_call to __imx_sc_rpc_call_flags
* Make a tiny imx_sc_rpc_call wrapper which just converts resp/noresp to 
a flag
* Make get button status call __imx_sc_rpc_call_flags with the 
_IMX_SC_RPC_NOERROR flag

Hope this makes my suggestion clearer? Pushing this to the caller is a 
bit ugly but I think it's worth preserving the fact that the imx rpc 
core treats services in an uniform way.

>>> Adding a special api shouldn't be the right fix. Imagine if someone
>>> (not a nxp-developer) wants to add a new driver. How could he be
>>> expected to know which api he should use. The better abbroach would be
>>> to fix the scu-fw instead of adding quirks..
> 
> Yes, fixing SCU FW is the best solution, but we have talked to SCU FW owner, the SCU
> FW released has been finalized, so the API implementation can NOT be changed, but
> they will pay attention to this issue for new added APIs later. That means the number
> of APIs having this issue a very limited.
> 
>>
>> Right now developers who want to make SCFW calls in upstream need to
>> define the message struct in their driver based on protocol documentation.
>> This includes:
>>
>> * Binary layout of the message (a packed struct)
>> * If the message has a response (already a bool flag)
>> * If an error code is returned (this patch adds support for it)
>>
>> Since callers are already exposed to the binary protocol exposing them to
>> minor quirks of the calling convention also seems reasonable. Having the
>> low-level IPC code peek at message IDs seems like a hack; this belong at a
>> slightly higher level.
> 
> A little confused, so what you suggested is to add make the imx_scu_call_rpc()
> becomes the "slightly higher level" API, then in this API, check the message IDs
> to decide whether to return error value, then calls a new API which will have
> the low-level IPC code, the this new API will have a flag passed from imx_scu_call_rpc()
> function, am I right?

No, I am saying that the caller (button driver) should be responsible 
for calling with a special flag which states that the RPC call.

In internal tree this is handled inside the machine-generated function 
calls, right? These are mostly skipped in upstream but maybe for these 
particular calls we can manually add wrappers inside 
"drivers/firmware/imx/misc.c".

And even if the functions "return void" from a SCFW perspective it still 
makes sense to return general kernel errors, for example from mailbox.

--
Regards,
Leonard