Re: [PATCH V2 2/4] drivers: qcom: rpmh-rsc: avoid locking in the interrupt handler

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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@xxxxxxxxxxxxxx>

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,

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