linux-kernel - Re: [RFC] dt-bindings: mailbox: add doorbell support to ARM MHU

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Message-ID: <CABb+yY30nmbBUzYG62xGEbrr7107h_Edyq3jKPheZAQ0Cvr9Yg@mail.gmail.com>
Date:   Mon, 18 May 2020 23:05:03 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] dt-bindings: mailbox: add doorbell support to ARM MHU

On Mon, May 18, 2020 at 10:40 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 18-05-20, 18:29, Bjorn Andersson wrote:
> > On Thu 14 May 22:17 PDT 2020, Viresh Kumar wrote:
> > > This stuff has been doing rounds on the mailing list since several years
> > > now with no agreed conclusion by all the parties. And here is another
> > > attempt to get some feedback from everyone involved to close this once
> > > and for ever. Your comments will very much be appreciated.
> > >
> > > The ARM MHU is defined here in the TRM [1] for your reference, which
> > > states following:
> > >
> > >     "The MHU drives the signal using a 32-bit register, with all 32
> > >     bits logically ORed together. The MHU provides a set of
> > >     registers to enable software to set, clear, and check the status
> > >     of each of the bits of this register independently.  The use of
> > >     32 bits for each interrupt line enables software to provide more
> > >     information about the source of the interrupt. For example, each
> > >     bit of the register can be associated with a type of event that
> > >     can contribute to raising the interrupt."
> > >
> >
> > Does this mean that there are 32 different signals and they are all ORed
> > into the same interrupt line to trigger software action when something
> > happens?
> >
> > Or does it mean that this register is used to pass multi-bit information
> > and when any such information is passed an interrupt will be triggered?
> > If so, what does that information mean? How is it tied into other Linux
> > drivers/subsystems?
>
> I have started to believe the hardware is written badly at this point
> :)
>
H/W is actually fine :)   Its just that the driver is written to
_also_ support a platform (my original) that doesn't have shmem and
need to pass data via 32bit registers.
Frankly, I am not against the doorbell mode, I am against implementing
two modes in a driver. If it really helped (note the past tense) the
SCMI, we could implement the driver only in doorbell mode but
unfortunately SCMI would still be _broken_ for non-doorbell
controllers.