linux-kernel - Re: [PATCH v9 5/9] mailbox: add CIX mailbox driver

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Message-ID: <aHcOaas459jctIB2@gchen>
Date: Wed, 16 Jul 2025 02:28:57 +0000
From: Guomin chen <guomin.chen@...tech.com>
To: Jassi Brar <jassisinghbrar@...il.com>, Arnd Bergmann <arnd@...db.de>
Cc: Rob Herring <robh@...nel.org>, krzk+dt@...nel.org,
	Conor Dooley <conor+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Peter Chen <peter.chen@...tech.com>,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, cix-kernel-upstream@...tech.com,
	Marc Zyngier <maz@...nel.org>, Sudeep Holla <sudeep.holla@....com>,
	Kajetan Puchalski <kajetan.puchalski@....com>,
	Enric Balletbo <eballetb@...hat.com>,
	Gary Yang <gary.yang@...tech.com>,
	Lihua Liu <Lihua.Liu@...tech.com>
Subject: Re: [PATCH v9 5/9] mailbox: add CIX mailbox driver

On Tue, Jul 15, 2025 at 05:11:01PM -0500, Jassi Brar wrote:
> [Some people who received this message don't often get email from jassisinghbrar@...il.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> 
> EXTERNAL EMAIL
> 
> On Mon, Jul 14, 2025 at 10:40 AM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Sun, Jul 13, 2025, at 19:00, Jassi Brar wrote:
> > > On Tue, Jul 8, 2025 at 8:54 AM Guomin chen <guomin.chen@...tech.com> wrote:
> > > ....
> > >> > > +/* [0~7] Fast channel
> > >> > > + * [8] doorbell base channel
> > >> > > + * [9]fifo base channel
> > >> > > + * [10] register base channel
> > >> > > + */
> > >> > > +#define MBOX_FAST_IDX          7
> > >> > > +#define MBOX_DB_IDX            8
> > >> > > +#define MBOX_FIFO_IDX          9
> > >> > > +#define MBOX_REG_IDX           10
> > >> > > +#define CIX_MBOX_CHANS         11
> > >> > > +
> > >> > if it is not really a single controller owning different channels,
> > >> > maybe implement only what you currently use.
> > >> >
> > >> As mentioned in the previous email, a single controller can support
> > >> multiple different channels.
> > >>
> > > OK. I am not too worried about having all variants in one driver esp
> > > when it is manageable and share the code.
> > > Unless I am overlooking something. Arnd?
> >
> > My main worry here is that the types are all quite different: while
> > the doorbell and fast mailboxes are what a lot of other drivers have,
> > the FIFO mode does not seem to be a good fit for the mailbox subsystem
> > but instead looks like a more generic firmware interface with variable
> > length messages.
> >
> > For those, I think a higher-level driver with fixed data structures
> > passed through the hardware interface seems more appropriate.
> >
> Yes. But sometimes when the data structures of a protocol are not
> bigger than FIFO depth, the platform may choose to use the FIFO mode.
> I see it as platform dependent.
> 
> > Are there any other mailbox drivers that just use the mailbox to
> > tunnel variable-length messages?
> >
> From a quick look, Armada 37xx and Hi6220 have fifo though they fill
> them up fully for each transfer.
>
Yes, both Armada 37xx and Hi6220 support FIFO functionality, and they
fill the FIFO with each transfer. 

Since the cix mailbox hardware supports messages with a maximum length
of 128 bytes, different clients transmit messages of varying lengths,
such as the cix DSP using 8 bytes, the cix sensorhub using 12 bytes, etc. 

Therefore, the cix mailbox driver has been modified to support variable-
length messages of up to 128 bytes. This allows for more compact and 
flexible support of various clients.

Thanks
Guomin Chen