Re: [PATCH 1/4] mailbox: arm_mhuv2: add device tree binding documentation

From: Tushar Khandelwal <Tushar.Khandelwal@arm.com>
To: Sudeep Holla <Sudeep.Holla@arm.com>,
	Jassi Brar <jassisinghbrar@gmail.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"tushar.2nov@gmail.com" <tushar.2nov@gmail.com>,
	"morten_bp@live.dk" <morten_bp@live.dk>, nd <nd@arm.com>,
	Morten Petersen <Morten.Petersen@arm.com>,
	Rob Herring <robh+dt@kernel.org>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Devicetree List <devicetree@vger.kernel.org>
Subject: Re: [PATCH 1/4] mailbox: arm_mhuv2: add device tree binding documentation
Date: Wed, 7 Aug 2019 11:11:19 +0000	[thread overview]
Message-ID: <9D863536-33C8-4103-A553-64C95FF94FC4@arm.com> (raw)
In-Reply-To: <20190802105357.GF23424@e107155-lin>

On 02/08/2019, 11:54, "Sudeep Holla" <sudeep.holla@arm.com> wrote:

    On Thu, Jul 25, 2019 at 12:49:58AM -0500, Jassi Brar wrote:
    > On Sun, Jul 21, 2019 at 4:58 PM Jassi Brar <jassisinghbrar@gmail.com> wrote:
    > >

    [...]

    > > If the mhuv2 instance implements, say, 3 channel windows between
    > > sender (linux) and receiver (firmware), and Linux runs two protocols
    > > each requiring 1 and 2-word sized messages respectively. The hardware
    > > supports that by assigning windows [0] and [1,2] to each protocol.
    > > However, I don't think the driver can support that. Or does it?
    > >
    > Thinking about it, IMO, the mbox-cell should carry a 128 (4x32) bit
    > mask specifying the set of windows (corresponding to the bits set in
    > the mask) associated with the channel.
    > And the controller driver should see any channel as associated with
    > variable number of windows 'N', where N is [0,124]
    >
    > mhu_client1: proto1@2e000000 {
    >        .....
    >        mboxes = <&mbox 0x0 0x0 0x0 0x1>
    > }
    >
    > mhu_client2: proto2@2f000000 {
    >        .....
    >        mboxes = <&mbox 0x0 0x0 0x0 0x6>
    > }
    >

    This still doesn't address the overhead I mentioned in my arm_mhu_v1
    series.

    As per you suggestion, we will have one channel with all possible
    bit mask value to specify the window. Let's imagine that 2 protocols
    share the same channel, then the requests are serialised.
    E.g. if bits 0 and 1 are allocated for say protocol#1 and bits 2 and 3
    for protocol#2.

At a given time only one protocol can be used by a client. No mix-match
of protocols are handled by the driver currently. Also its not possible to address all
possible scenarios offered by the IP. That's why the current driver design is
based on the implementation in the existing platforms.

    Further protocol#1 has higher latency requirements like sched-governor
    DVFS and there are 3-4 pending requests on protocol#2, then the incoming
    requests for protocol#1 is blocked.

    This is definitely overhead and I have seen lots of issue around this
    and hence I was requesting that we need to create individual channels
    for each of these. Having abstraction on top to multiplex or arbitrate
    won't help.

Also the (mbox-cells) approach will not allow us to differentiate between
single-word and doorbell which is required to make the controller driver
aware of the data expected whether it's a pointer to a location or in
register itself.

--Tushar
    --
    Regards,
    Sudeep

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