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Date: Fri, 13 Jan 2023 14:16:20 -0800
From: Guru Das Srinagesh <quic_gurus@...cinc.com>
To: Sibi Sankar <quic_sibis@...cinc.com>
CC: <andersson@...nel.org>, <srinivas.kandagatla@...aro.org>,
<agross@...nel.org>, <linux-arm-msm@...r.kernel.org>,
<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<krzysztof.kozlowski+dt@...aro.org>, <robh+dt@...nel.org>,
<konrad.dybcio@...ainline.org>, <robimarko@...il.com>
Subject: Re: [PATCH V10 2/2] firmware: qcom: scm: Add wait-queue handling
logic
On Jan 13 2023 21:41, Sibi Sankar wrote:
> From: Guru Das Srinagesh <quic_gurus@...cinc.com>
>
> When the firmware (FW) supports multiple requests per VM, multiple requests
> from the same/different VM can reach the firmware at the same time. Since
> the firmware currently being used has limited resources, it guards them
> with a resource lock and puts requests on a wait-queue internally and
> signals to HLOS that it is doing so. It does this by returning a new return
> value in addition to success or error: SCM_WAITQ_SLEEP. A sleeping SCM call
> can be woken up by an interrupt that the FW raises.
>
> 1) SCM_WAITQ_SLEEP:
>
> When an SCM call receives this return value instead of success
> or error, FW has placed this call on a wait-queue and has signalled
> HLOS to put it to non-interruptible sleep.
>
> Along with this return value, FW also passes to HLOS `wq_ctx` -
> a unique number (UID) identifying the wait-queue that it has put
> the call on, internally. This is to help HLOS with its own
> bookkeeping to wake this sleeping call later.
>
> Additionally, FW also passes to HLOS `smc_call_ctx` - a UID
> identifying the SCM call thus being put to sleep. This is also
> for HLOS' bookkeeping to wake this call up later.
>
> These two additional values are passed via the a1 and a2
> registers.
>
> N.B.: The "ctx" in the above UID names = "context".
>
> The handshake mechanism that HLOS uses to talk to FW about wait-queue
> operations involves two new SMC calls.
>
> 1) get_wq_ctx():
>
> Arguments: None
> Returns: wq_ctx, flags, more_pending
>
> Get the wait-queue context, and wake up either one or all of the
> sleeping SCM calls associated with that wait-queue.
>
> Additionally, repeat this if there are more wait-queues that are
> ready to have their requests woken up (`more_pending`).
>
> 2) wq_resume(smc_call_ctx):
>
> Arguments: smc_call_ctx
>
> HLOS needs to issue this in response to receiving an
> IRQ, passing to FW the same smc_call_ctx that FW
> receives from HLOS via the get_wq_ctx() call.
>
> (The mechanism to wake a SMC call back up is described in detail below)
>
> VM_1 VM_2 Firmware
> │ │ │
> │ │ │
> │ │ │
> │ │ │
> │ REQUEST_1 │ │
> ├────────────────────────┼─────────────────────────────────┤
> │ │ │
> │ │ ┌──┼──┐
> │ │ │ │ │
> │ │ REQUEST_2 │ │ │
> │ ├──────────────────────────────┼──┤ │
> │ │ │ │ │Resource
> │ │ │ │ │is busy
> │ │ {WQ_SLEEP} │ │ │
> │ │◄─────────────────────────────┼──┤ │
> │ │ wq_ctx, smc_call_ctx │ │ │
> │ │ └──┼──┘
> │ REQUEST_1 COMPLETE │ │
> │◄───────────────────────┼─────────────────────────────────┤
> │ │ │
> │ │ IRQ │
> │ │◄─-------------------------------│
> │ │ │
> │ │ get_wq_ctx() │
> │ ├────────────────────────────────►│
> │ │ │
> │ │ │
> │ │◄────────────────────────────────┤
> │ │ wq_ctx, flags, and │
> │ │ more_pending │
> │ │ │
> │ │ │
> │ │ wq_resume(smc_call_ctx) │
> │ ├────────────────────────────────►│
> │ │ │
> │ │ │
> │ │ REQUEST_2 COMPLETE │
> │ │◄────────────────────────────────┤
> │ │ │
> │ │ │
>
> With the exception of get_wq_ctx(), the other SMC call wq_resume() can
> return WQ_SLEEP (these nested rounds of WQ_SLEEP are not shown in the
> above diagram for the sake of simplicity). Therefore, introduce a new
> do-while loop to handle multiple WQ_SLEEP return values for the same
> parent SCM call.
>
> Request Completion in the above diagram refers to either a success
> return value (zero) or error (and not SMC_WAITQ_SLEEP)
>
> Also add the interrupt handler that wakes up a sleeping SCM call.
>
> Signed-off-by: Guru Das Srinagesh <quic_gurus@...cinc.com>
> Co-developed-by: Sibi Sankar <quic_sibis@...cinc.com>
> Signed-off-by: Sibi Sankar <quic_sibis@...cinc.com>
Reviewed-by: Guru Das Srinagesh <quic_gurus@...cinc.com>
Thank you.
Guru Das.
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