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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 <sudeep.holla@arm.com>
>
> 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
possible.

Rob

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