RE: [PATCH v7 3/6] dt-bindings: mailbox: imx-mu: add generic MU channel support

["A.s. Dong" <aisheng.dong@nxp.com>]

> -----Original Message-----
> From: Jassi Brar [mailto:jassisinghbrar@gmail.com]
> Sent: Friday, July 27, 2018 12:56 PM
> To: A.s. Dong <aisheng.dong@nxp.com>
> Cc: Oleksij Rempel <o.rempel@pengutronix.de>; , Sascha Hauer
> <kernel@pengutronix.de>; Shawn Guo <shawnguo@kernel.org>; Fabio
> Estevam <fabio.estevam@nxp.com>; Rob Herring <robh+dt@kernel.org>;
> Mark Rutland <mark.rutland@arm.com>; Vladimir Zapolskiy
> <vladimir_zapolskiy@mentor.com>; , linux-arm-kernel@lists.infradead.org,
> linux-mediatek@lists.infradead.org, srv_heupstream <linux-arm-
> kernel@lists.infradead.org>; Devicetree List <devicetree@vger.kernel.org>;
> dl-linux-imx <linux-imx@nxp.com>
> Subject: Re: [PATCH v7 3/6] dt-bindings: mailbox: imx-mu: add generic MU
> channel support
> 
> On Fri, Jul 27, 2018 at 9:32 AM, A.s. Dong <aisheng.dong@nxp.com> wrote:
> >>
> >> >>
> >> >> > Each MU has four pairs of rx/tx data register with four rx/tx
> >> >> > interrupts which can also be used as a separate channel.
> >> >> >
> >> >> So the hardware actually supports 4 channels.
> >> >>
> >> >> > -- #mbox-cells:  Must be 0. Number of cells in a mailbox
> >> >> > +- #mbox-cells:  Must be:
> >> >> > +               0 - for single channel mode. i.MX8* SCU protocol specific.
> >> >> > +               1 - for multichannel (generic) mode.
> >> >> > +
> >> >> No, please.
> >> >> DT bindings should reflect the real hardware, and not the software
> >> >> mode we want the driver to work in.
> >> >> Please define mbox-cells=1  and have the i.MX8* platform always
> >> >> ask for channel-0.
> >> >
> >> > The reality is that MU hardware does not define channels in
> >> > reference
> >> manual.
> >> > However, it does have four separate data tx/rx register which can
> >> > be used as 'virtual' channels which is supported by this current driver.
> >> >
> >> > See below HW description from the reference manual:
> >> > For messaging, the MU has four, 32-bit write-only transmit
> >> > registers and four, 32-bit read-only receive registers on the
> >> > Processor B and Processor A-sides. These registers are used for
> >> > sending messages to each
> >> other.
> >> >
> >> For a while please forget the protocol(user) level usage, and
> >> consider only what your h/w is.
> >>
> >> MU has 4 pairs of TX_Reg + TX_IRQ, and 4 pairs of RX_Reg + RX_IRQ.
> >> (MU also has 4 "doorbell" type channels that it calls GP, but those
> >> are not managed here, so lets not worry atm).
> >>
> >> By definition, a mailbox channel is simply a signal, optionally with
> >> data attached. So, MU has 4 TX and 4 RX channels.
> >>
> >> The MU driver should populate 8 unidirectional (4 Tx and 4 RX)
> >> channels and set each tx/rx operation to trigger the corresponding
> >> interrupt. This is not my whim, this is how the controller is!
> >>
> >
> > This looks like reasonable to me, theoretically.
> > Just not sure whether it's necessary to support it because we probably
> > will never use like that in reality, then it might become meaningless
> > complicity introduced and error prone.
> >
> It _is_ necessary to write controller driver independent of client drivers.
> 

Yes, that's true. What if we think we're writing driver against HW capabilities
which support 4 pair of channel links(tx/rx)? 
It looks like independent of client drivers and may make life easily.
Does it make sense?

> 
> > And AFAIK ARM MHU is doing the same way as MU which looks like also
> unidirectional channel.
> >
> https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Finfo
> >
> center.arm.com%2Fhelp%2Findex.jsp%3Ftopic%3D%2Fcom.arm.doc.ddi0515
> b%2F
> >
> CHDGBGIF.html&amp;data=02%7C01%7Caisheng.dong%40nxp.com%7Cb728
> 16362983
> >
> 4208f9e908d5f37d3e00%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%
> 7C6366
> >
> 82641593785009&amp;sdata=VsG0oXxEObPRwT5KVss2eZthSTMTR2%2BMrv
> PqhDUpPGU
> > %3D&amp;reserved=0
> > drivers/mailbox/arm_mhu.c
> >
> MHU driver is doing exactly what the data sheet says. I know because I wrote
> the driver.
> 

Hmm... Maybe I missed something, but seems no from what I see:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0922g/CCHHGIAH.html

Let's see the data sheet:
2.1.1. Physical channels
The CSS MHU peripheral provides physical channels that are used for communication between
the SCP and the AP.  These channels are called physical channels because their implementation
is fixed in hardware.  Each physical channel is unidirectional (SCP to AP or AP to SCP).

A physical channel comprises:
A 32-bit STAT (STATUS) register:
A 32-bit SET register:
A 32-bit CLEAR register:

The driver seems composes them into a pair of links with both tx and rx physical channel.
drivers/mailbox/arm_mhu.c
#define MHU_CHANS       3

struct mhu_link {
        unsigned irq;
        void __iomem *tx_reg;
        void __iomem *rx_reg;
};

for (i = 0; i < MHU_CHANS; i++) {
        mhu->chan[i].con_priv = &mhu->mlink[i];
        mhu->mlink[i].irq = adev->irq[i];
        mhu->mlink[i].rx_reg = mhu->base + mhu_reg[i];
        mhu->mlink[i].tx_reg = mhu->mlink[i].rx_reg + TX_REG_OFFSET;
}
And the driver totally supports 3 links lp, hp and sec. Each links actually
are using two unidirectional physical channels. Am I correct?

> 
> >> The SCU is poorly implemented as it ignores 3 irqs and club all 4
> >> register together (there are many other cons of this approach but
> >> lets not get into that).
> >> Personally, I would push-back on such a bad design. But if you claim
> >> you have no choice but to support SCU as such, the work around could
> >> be simpler than defining a new "scu mode" altogether.
> >>
> >
> > This is one of the recommended ways designed in HW reference manual
> > and it allows to send frame information up to 4 words without using shared
> memory.
> > SCU just follows it, so it's hard to believe it's a bad design.
> >
> >> #mbox-cells:  Must be 3.
> >>                       First cell is 1 for TX and 0 for RX channel
> >>                       Second cell is index of the channel [0,1,2 or 3]
> >>                       Third cell is 1 if the channel triggers an IRQ,
> >> 0 if not. That is ACR.RIE/TIE bits are set or not.
> >>
> >> Normal clients would always request a channel with irqs enabled.
> >> The SCU client would request all 8 channels -- TX/RX[0,1,2] with irqs
> >> disabled, TX/RX[3] with irqs enabled. And SCU will read/write 4 word
> >> data over 4 rx/tx channels, instead of the virtually concocted one.
> >>
> >
> > It may work If SCU protocol data length is fixed to 4 words. However,
> > it's length is flexible for different SVC service. That means this binding
> won't work for SCU.
> > And it will introduce much complexities during the implementation.
> >
> > Instead, we're using polling mode for both TX/RX and the data size is
> > stored in the msg header and sending msgs using all 4 data tx/rx registers
> as a channel fifo.
> >
> You first give me the "Passing short messages" usecase (quite a bad
> one) and ask how it can work. When I give you a solution, you effectively say
> "well, my usecase isn't even that". I feel I just wasted my time.
> 

Sorry for may not clear, "Passing short message' usecase is to tell how the
HW is working on one channel mode sending up to 4 words in one time
As specified in reference manual.

SCU does work that way, the only difference is it's using polling mode
rather than interrupt driven.  The point is the data size may be different
for each msg, so we can't simply know which data register interrupt
should be enable from static data defined in device tree.

> 
> >>
> >> > short messages
> >> > Transmit register(s) can be used to pass short messages from one to
> >> > four words in length. For example, when a four-word message is
> >> > desired, only one of the registers needs to have its corresponding
> >> > interrupt enable bit set at the receiver side; the message’s first
> >> > three words are written to the registers whose interrupt is masked,
> >> > and the fourth word is written to the other register (which
> >> > triggers an
> >> interrupt at the receiver side).
> >> >
> >> > The reference manual is at here: (Chapter 42 Messaging Unit (MU)
> >> >
> >>
> https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fw
> >> ww
> >> > .nxp.com%2Fdocs%2Fen%2Freference-
> >> manual%2FIMX6ULRM.pdf&amp;data=02%7C0
> >> >
> >>
> 1%7Caisheng.dong%40nxp.com%7Cef349d7c9caf46c4260008d5f30e9ef5%7C6
> >> 86ea1
> >> >
> >>
> d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C636682166494860332&amp;sdat
> >> a=54rE
> >> >
> >>
> iDm%2BGD6EY8NE64ck1LXVGoUCtWYrHXPoWqcujUo%3D&amp;reserved=0
> >> >
> >> > And SCU firmware does use MU in above way specified by reference
> >> manual.
> >> > Even from HW point of view, it's still one channel only mailbox.
> >> >
> >> Please realise that any manual is written by a mere mortal afterall.
> >> How the controller works is set in stone, but how the controller can
> >> be used ... is just someones suggestion.
> >>
> >> The approach I suggest above, conforms to the api and prevents a
> >> provider dancing to the tunes of a user.
> >
> > First of all, really appreciate for your suggestions.
> > It may be hard to find a common binding with the same mbox-cells.
> > It looks like we just need a property is distinguish how the Mailbox
> > is used In one channel or multi channel mode.
> >
> I get the idea you were ready to see the code merged in the coming window
> and be done with it. And now it feels lazy to change the code.

For me, I'm glad to change if we have a clear better solution.
And I do appreciate your suggestion and review time.

Why I'm a bit hesitate now is because your suggestion may not work for SCU,
(see above explanation), but it does work for generic M4 case.
But we' re now going to support both protocol in one mailbox driver.
Any suggestion on how to treat them properly if change the binding?

> I am sorry, but I can't allow controller drivers "emulating" some mode for a
> client driver. That is moving a part of client code into the controller driver.
> 

Okay, let's figure out it first.
Would you be more specific on what "emulating" did you mean in controller driver?
Sending up to 4 words capability in one channel mode as specified in reference manual?
That's I'm a bit confusing because I thought it's HW capability independent of client driver.

Or anything else?

> 
> > Directly checking mbox-cells seems the most easy way and it is already
> > Acked by Rob. Can't this way be Okay to you?
> >
> Rob is indeed far better than I am. But he also has to look into 100 other
> subsystems, whereas I am only looking into mailbox. I have time to dig into
> your datasheets. I believe Rob will point out anything wrong I suggest.
> 

Yes, you're in the fair enough authority to judge it. Thanks for your effort.

> BTW, the submitted driver doesn't even support the SCU usecase. Why the
> bindings?

Because that patch is firstly Acked by Rob. Others are reworked and ready to be sent
out against this patch series. But it seems we still have unresolved issues now as you
pointed out. We can first resolve them. Or do you need me to send out for your reference?

Regards
Dong Aisheng


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