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    SubjectRe: [RFC] dt-bindings: mailbox: add doorbell support to ARM MHU
    On Fri, May 15, 2020 at 10:47:38AM +0530, Viresh Kumar wrote:
    > From: Sudeep Holla <>
    > Hi Rob, Arnd and Jassi,
    > 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."
    > On few other platforms, like qcom, similar doorbell mechanism is present
    > with separate interrupt for each of the bits (that's how I understood
    > it), while in case of ARM MHU, there is a single interrupt line for all
    > the 32 bits. Also in case of ARM MHU, these registers and interrupts
    > have 3 copies for different priority levels, i.e. low priority
    > non-secure, high priority non-secure and secure channels.
    > For ARM MHU, both the dt bindings and the Linux driver support 3
    > channels for the different priorities right now and support sending a 32
    > bit data on every transfer in a locked fashion, i.e. only one transfer
    > can be done at once and the other have to wait for it to finish first.
    > Here are the point of view of the parties involved on this subject:
    > Jassi's viewpoint:
    > - Virtualization of channels should be discouraged in software based on
    > specific usecases of the application. This may invite other mailbox
    > driver authors to ask for doing virtualization in their drivers.
    > - In mailbox's terminology, every channel is equivalent to a signal,
    > since there is only one signal generated here by the MHU, there should
    > be only one channel per priority level.
    > - The clients should send data (of just setting 1 bit or many in the 32
    > bit word) using the existing mechanism as the delays due to
    > serialization shouldn't be significant anyway.
    > - The driver supports 90% of the users with the current implementation
    > and it shouldn't be extended to support doorbell and implement two
    > different modes by changing value of #mbox-cells field in bindings.
    > Sudeep (ARM) and myself as well to some extent:
    > - The hardware gives us the capability to write the register in
    > parallel, i.e. we can write 0x800 and 0x400 together without any
    > software locks, and so these 32 bits should be considered as separate
    > channel even if only one interrupt is issued by the hardware finally.
    > This shouldn't be called as virtualization of the channels, as the
    > hardware supports this (as clearly mentioned in the TRM) and it takes
    > care of handling the signal properly.
    > - With serialization, if we use only one channel as today at every
    > priority, if there are 5 requests to send signal to the receiver and
    > the dvfs request is the last one in queue (which may be called from
    > scheduler's hot path with fast switching), it unnecessarily needs to
    > wait for the first four transfers to finish due to the software
    > locking imposed by the mailbox framework. This adds additional delay,
    > maybe of few ms only, which isn't required by the hardware but just by
    > the software and few ms can be important in scheduler's hotpath.
    > - With the current approach it isn't possible to assign different bits
    > (or doorbell numbers) to clients from DT and the only way of doing
    > that without adding new bindings is by extending #mbox-cells to accept
    > a value of 2 as done in this patch.
    > Jassi and Sudeep, I hope I was able to represent both the view points
    > properly here. Please correct me if I have made a mistake here.
    > This is it. It would be nice to get the views of everyone now on this
    > and how should this be handled.

    I am perfectly fine with adding another cell which seems appropriate
    here. You can have 5 cells for all I care if that makes sense for
    the h/w. That has nothing to do with the Linux design. Whether Linux
    requires serializing mailbox accesses is a separate issue. On that side,
    it seems silly to not allow driving the h/w in the most efficient way


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