linux-kernel - Re: [PATCH v8 09/28] mailbox: Add Gunyah message queue mailbox

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Message-ID: <65af5abc-3719-95fe-041c-67d1d146aba1@linaro.org>
Date:   Tue, 17 Jan 2023 13:21:13 -0600
From:   Alex Elder <elder@...aro.org>
To:     Elliot Berman <quic_eberman@...cinc.com>,
        Alex Elder <elder@...aro.org>,
        Bjorn Andersson <quic_bjorande@...cinc.com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Murali Nalajala <quic_mnalajal@...cinc.com>,
        Jonathan Corbet <corbet@....net>
Cc:     Trilok Soni <quic_tsoni@...cinc.com>,
        Srivatsa Vaddagiri <quic_svaddagi@...cinc.com>,
        Carl van Schaik <quic_cvanscha@...cinc.com>,
        Prakruthi Deepak Heragu <quic_pheragu@...cinc.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v8 09/28] mailbox: Add Gunyah message queue mailbox

On 1/10/23 12:16 PM, Elliot Berman wrote:
>>> +    ret = gh_hypercall_msgq_send(msgq->tx_ghrsc->capid, 
>>> msgq_data->length,
>>> +                    (uintptr_t)msgq_data->data, tx_flags, &ready);
>>> +
>>> +    /**
>>> +     * unlikely because Linux tracks state of msgq and should not 
>>> try to
>>> +     * send message when msgq is full.
>>> +     */
>>
>> Is it just unlikely, or is it impossible?
>>
> 
> This would require multiple mailbox controllers interacting with the 
> same message queue.
> 
> The only way I can think this is possible is if the Gunyah drivers are 
> unloaded when the message queue is full; drivers are then re-loaded 
> before the receiver processes the messages. The initial internal state 
> of the message queue controller assumes that there is space in the 
> message queue. We would get a Tx vIRQ once space becomes available and 
> the message would then be attempted to sent again. Since there's a safe 
> flow to recover from a inconsistent internal state and it's very 
> unlikely to start in that state, I don't think we need to add calls to 
> check if the message queue is full during initialization.

First, your explanation is appreciated but more than what I
was looking for...  I just mentioned it because if it were
impossible, then there's no point in having this code handle
something that literally can't happen.

But as far as your explanation, I *hope* if the Gunyah drivers
are unloaded, everything gets fully cleaned up before that
completes.  There should be no in-flight activity, or
any "previous generation" messages that could be processed.

In any case, I think my question is answered.  Thanks.

					-Alex