From: Lina Iyer <ilina@codeaurora.org>
To: Stephen Boyd <swboyd@chromium.org>
Cc: agross@kernel.org, bjorn.andersson@linaro.org,
linux-arm-msm@vger.kernel.org, linux-soc@vger.kernel.org,
rnayak@codeaurora.org, linux-kernel@vger.kernel.org,
linux-pm@vger.kernel.org, dianders@chromium.org,
mkshah@codeaurora.org
Subject: Re: [PATCH V2 2/4] drivers: qcom: rpmh-rsc: avoid locking in the interrupt handler
Date: Wed, 24 Jul 2019 08:52:51 -0600 [thread overview]
Message-ID: <20190724145251.GB18620@codeaurora.org> (raw)
In-Reply-To: <5d3769df.1c69fb81.55d03.aa33@mx.google.com>
On Tue, Jul 23 2019 at 14:11 -0600, Stephen Boyd wrote:
>Quoting Lina Iyer (2019-07-22 14:53:38)
>> Avoid locking in the interrupt context to improve latency. Since we
>> don't lock in the interrupt context, it is possible that we now could
>> race with the DRV_CONTROL register that writes the enable register and
>> cleared by the interrupt handler. For fire-n-forget requests, the
>> interrupt may be raised as soon as the TCS is triggered and the IRQ
>> handler may clear the enable bit before the DRV_CONTROL is read back.
>>
>> Use the non-sync variant when enabling the TCS register to avoid reading
>> back a value that may been cleared because the interrupt handler ran
>> immediately after triggering the TCS.
>>
>> Signed-off-by: Lina Iyer <ilina@codeaurora.org>
>> ---
>
>I have to read this patch carefully. The commit text isn't convincing me
>that it is actually safe to make this change. It mostly talks about the
>performance improvements and how we need to fix __tcs_trigger(), which
>is good, but I was hoping to be convinced that not grabbing the lock
>here is safe.
>
>How do we ensure that drv->tcs_in_use is cleared before we call
>tcs_write() and try to look for a free bit? Isn't it possible that we'll
>get into a situation where the bitmap is all used up but the hardware
>has just received an interrupt and is going to clear out a bit and then
>an rpmh write fails with -EBUSY?
>
If we have a situation where there are no available free bits, we retry
and that is part of the function. Since we have only 2 TCSes avaialble
to write to the hardware and there could be multiple requests coming in,
it is a very common situation. We try and acquire the drv->lock and if
there are free TCS available and if available mark them busy and send
our requests. If there are none available, we keep retrying.
>> drivers/soc/qcom/rpmh-rsc.c | 4 +---
>> 1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/soc/qcom/rpmh-rsc.c b/drivers/soc/qcom/rpmh-rsc.c
>> index 5ede8d6de3ad..694ba881624e 100644
>> --- a/drivers/soc/qcom/rpmh-rsc.c
>> +++ b/drivers/soc/qcom/rpmh-rsc.c
>> @@ -242,9 +242,7 @@ static irqreturn_t tcs_tx_done(int irq, void *p)
>> write_tcs_reg(drv, RSC_DRV_CMD_ENABLE, i, 0);
>> write_tcs_reg(drv, RSC_DRV_CMD_WAIT_FOR_CMPL, i, 0);
>> write_tcs_reg(drv, RSC_DRV_IRQ_CLEAR, 0, BIT(i));
>> - spin_lock(&drv->lock);
>> clear_bit(i, drv->tcs_in_use);
>> - spin_unlock(&drv->lock);
>> if (req)
>> rpmh_tx_done(req, err);
>> }
>> @@ -304,7 +302,7 @@ static void __tcs_trigger(struct rsc_drv *drv, int tcs_id)
>> enable = TCS_AMC_MODE_ENABLE;
>> write_tcs_reg_sync(drv, RSC_DRV_CONTROL, tcs_id, enable);
>> enable |= TCS_AMC_MODE_TRIGGER;
>> - write_tcs_reg_sync(drv, RSC_DRV_CONTROL, tcs_id, enable);
>> + write_tcs_reg(drv, RSC_DRV_CONTROL, tcs_id, enable);
>> }
>>
>> static int check_for_req_inflight(struct rsc_drv *drv, struct tcs_group *tcs,
next prev parent reply other threads:[~2019-07-24 14:52 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-22 21:53 [PATCH V2 1/4] drivers: qcom: rpmh-rsc: simplify TCS locking Lina Iyer
2019-07-22 21:53 ` [PATCH V2 2/4] drivers: qcom: rpmh-rsc: avoid locking in the interrupt handler Lina Iyer
2019-07-23 20:11 ` Stephen Boyd
2019-07-24 14:52 ` Lina Iyer [this message]
2019-07-24 19:37 ` Stephen Boyd
2019-07-24 20:36 ` Lina Iyer
2019-07-24 23:27 ` Doug Anderson
2019-07-25 15:18 ` Lina Iyer
2019-07-25 15:39 ` Doug Anderson
2019-07-29 19:01 ` Lina Iyer
2019-07-29 20:56 ` Stephen Boyd
2019-07-30 17:29 ` Lina Iyer
2019-07-22 21:53 ` [PATCH V2 3/4] drivers: qcom: rpmh: switch over from spinlock irq variants Lina Iyer
2019-07-23 18:24 ` Stephen Boyd
2019-07-22 21:53 ` [PATCH V2 4/4] drivers: qcom: rpmh-rsc: remove redundant register access Lina Iyer
2019-07-23 18:22 ` [PATCH V2 1/4] drivers: qcom: rpmh-rsc: simplify TCS locking Stephen Boyd
2019-07-23 19:21 ` Lina Iyer
2019-07-23 20:18 ` Stephen Boyd
2019-07-24 14:54 ` Lina Iyer
2019-07-24 18:32 ` Stephen Boyd
2019-07-24 19:36 ` Lina Iyer
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