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Date:   Mon, 30 Sep 2019 12:02:22 +0200
From:   Marco Felsch <m.felsch@pengutronix.de>
To:     Anson Huang <anson.huang@nxp.com>
Cc:     Aisheng Dong <aisheng.dong@nxp.com>,
        "festevam@gmail.com" <festevam@gmail.com>,
        "s.hauer@pengutronix.de" <s.hauer@pengutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        dl-linux-imx <linux-imx@nxp.com>,
        "kernel@pengutronix.de" <kernel@pengutronix.de>,
        Leonard Crestez <leonard.crestez@nxp.com>,
        "shawnguo@kernel.org" <shawnguo@kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] firmware: imx: Skip return value check for some special
 SCU firmware APIs
Message-ID: <20190930100222.p2cx6xspjeunsm54@pengutronix.de>
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Hi Anson,

On 19-09-30 08:32, Anson Huang wrote:
> Hi, Marco
> 
> > On 19-09-30 07:42, Anson Huang wrote:
> > > Hi, Leonard
> > >
> > > > 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.
> > >
> > > It is clear now, so essentially it is same as 2 separate APIs, still
> > > need to change the button driver and uid driver to use the special
> > > flag, meanwhile, need to change the third parament of imx_sc_rpc_call()
> > from bool to u32.
> > >
> > > If no one opposes this approach, I will redo the patch together with
> > > the button driver and uid driver after holiday.
> > 
> > As Ansons said that are two approaches and in both ways the caller needs to
> > know if the error code is valid. Extending the flags seems better to me but it
> > looks still not that good. One question, does the scu-fw set the error-msg to
> > something? If not than why should we specify a flag or a other api?
> > Nowadays the caller needs to know that the error-msg-field isn't set so if the
> > caller sets the msg-packet to zero and fills the rpc-id the error-msg-field
> > shouldn't be touched by the firmware. So it should be zero.
> 
> The flow are as below for those special APIs with response data but no return value from SCU FW:
> 
> 1. caller sends msg with a header field and data field, the header field has svc ID and function ID;
> 2. SCU FW will service the caller and then clear the SVC ID before return, the response data will be
> Put in msg data field, and if the APIs has return value, SCU FW will put the return value in function ID of msg;  

Thanks for the declaration :)

> The caller has no chance to set the msg-packet to zero and rpc-id, it needs to pass correct rpc-id to SCU FW and
> Get response data from SCU FW, and for those special APIs has function ID NOT over-written by SCU FW's return
> Value, but the function ID is a unsigned int, and the SCU FW return value is also a unsigned int, so we have no
> idea to separate them for no-return value API or error-return API.

I see.

> With new approach, I can use below 2 flags, the ugly point is user need to know which API to call.

I don't see any improve using flags because the caller still needs to
know if the scu-fw works (sorry for that) correctly. So we should go to
adapt your approach to handle that within the core and improve the
caller usage.

What about this:

8<-------------------------------------------------------------------------------

diff --git a/drivers/firmware/imx/imx-scu.c b/drivers/firmware/imx/imx-scu.c
index 04a24a863d6e..8f406a0784a4 100644
--- a/drivers/firmware/imx/imx-scu.c
+++ b/drivers/firmware/imx/imx-scu.c
@@ -184,6 +184,16 @@ int imx_scu_call_rpc(struct imx_sc_ipc *sc_ipc, void *msg, bool have_resp)
 		/* response status is stored in hdr->func field */
 		hdr = msg;
 		ret = hdr->func;
+
+		/*
+		 * Some special SCU firmware APIs do NOT have return value
+		 * in hdr->func, but they do have response data, those special
+		 * APIs are defined as void function in SCU firmware, so they
+		 * should be treated as return success always.
+		 */
+		if (hdr->func == IMX_SC_MISC_FUNC_UNIQUE_ID ||
+		    hdr->func == IMX_SC_MISC_FUNC_GET_BUTTON_STATUS)
+			ret = 0;
 	}

 out:
8<-------------------------------------------------------------------------------

As you and Leonard said, this scu-fw behaviour is intended. So this will
be not changed over the time else we need a scu-fw version check too.
Also as you said those special functions shouldn't be extended I think a
simple if-statement should work and no performance regressions are
expected.

Regards,
  Marco

> +++ b/include/linux/firmware/imx/ipc.h
> @@ -35,6 +35,11 @@ struct imx_sc_rpc_msg {
>         uint8_t func;
>  };
> 
> +#define IMX_SC_RPC_HAVE_RESP   BIT(0) /* caller has response data */
> +#define IMX_SC_RPC_NOERROR     BIT(1) /* caller has response data but no return value from SCU FW */
> +
> +int imx_scu_call_rpc_flags(struct imx_sc_ipc *ipc, void *msg, u32 flags);
> 
> Anson
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

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