Re: [BUG] imx-sdma: readl_relaxed_poll_timeout_atomic() conversion - Russell King

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Lucas Stach <l.stach@pengutronix.de>
Cc: Michael Olbrich <m.olbrich@pengutronix.de>,
	Vinod Koul <vinod.koul@intel.com>,
	linux-arm-kernel@lists.infradead.org, dmaengine@vger.kernel.org
Subject: Re: [BUG] imx-sdma: readl_relaxed_poll_timeout_atomic() conversion
Date: Mon, 24 Jun 2019 13:15:33 +0100	[thread overview]
Message-ID: <20190624121533.3sd6mmxjfktllp2j@shell.armlinux.org.uk> (raw)
In-Reply-To: <1561378450.2587.3.camel@pengutronix.de>

On Mon, Jun 24, 2019 at 02:14:10PM +0200, Lucas Stach wrote:
> Hi Russell,
> 
> Am Samstag, den 22.06.2019, 19:42 +0100 schrieb Russell King - ARM Linux admin:
> > On Sat, Jun 22, 2019 at 08:10:29PM +0200, Michael Olbrich wrote:
> > > On Sat, Jun 22, 2019 at 05:53:18PM +0100, Russell King - ARM Linux admin wrote:
> > > > Old code:
> > > > 
> > > > -       while (!(ret = readl_relaxed(sdma->regs + SDMA_H_INTR) & 1)) {
> > > > -               if (timeout-- <= 0)
> > > > -                       break;
> > > > -               udelay(1);
> > > > -       }
> > > > 
> > > > So, while bit 0 is _clear_ the loop continues to poll.
> > > > 
> > > > 
> > > > New code:
> > > > 
> > > > +       ret = readl_relaxed_poll_timeout_atomic(sdma->regs + SDMA_H_STATSTOP,
> > > > +                                               reg, !(reg & 1), 1, 500);
> > > > 
> > > > Doesn't really tell us what the termination condition is (because of
> > > > the obfuscation taking away the details), but if we dig into the
> > > > macro maze:
> > > > 
> > > > #define readl_relaxed_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
> > > >         readx_poll_timeout_atomic(readl_relaxed, addr, val, cond, delay_us, timeout_us)
> > > > 
> > > > #define readx_poll_timeout_atomic(op, addr, val, cond, delay_us, timeout_us) \
> > > > ({ \
> > > >         u64 __timeout_us = (timeout_us); \
> > > >         unsigned long __delay_us = (delay_us); \
> > > >         ktime_t __timeout = ktime_add_us(ktime_get(), __timeout_us); \
> > > >         for (;;) { \
> > > >                 (val) = op(addr); \
> > > >                 if (cond) \
> > > >                         break; \
> > > > 
> > > > "cond" is passed in to here unmodified, so this becomes:
> > > > 
> > > > 	for (;;) {
> > > > > > > 		reg = readl_relaxed(sdma->regs + SDMA_H_STATSTOP);
> > > > > > > 		if (!(reg & 1))
> > > > > > > 			break;
> > > > 
> > > > So, if bit 0 of this register is clear, we terminate the loop.
> > > > 
> > > > Seems to me like this is a great illustration why using a helper
> > > > _introduces_ bugs, because it hides the detail about what the exit
> > > > condition for the embedded loop actually is, and leads to this kind
> > > > of error.
> > > > 
> > > > In any case, the conversion is obviously incorrect.
> > > > 
> > > > I occasionally see the "Timeout waiting for CH0 ready" error during
> > > > boot on a cbi4, which, given the above, means that we did end up
> > > > seeing bit 1 set (so according to the old code, we waited
> > > > successfully.)
> > > 
> > > The old code was polling SDMA_H_INTR so it waited for the bit to be set.
> > > The new code (as documented in the commit message) polls SDMA_H_STATSTOP
> > > instead.
> > > I believe this register is called SDMAARM_STOP_STAT in the reference
> > > manual. And the documentation states: "Reading this register yields the
> > > current state of the HE[i] bits".
> > > And from the documentation of the SDMA "DONE" instruction:
> > > "Clear HE bit for the current channel, send an interrupt to the Arm
> > > platform for the current channel and reschedule."
> > > 
> > > My interpretation of this is, that waiting for the bit in SDMA_H_STATSTOP
> > > to become zero has the same effect as waiting for the bit in SDMA_H_INTR to
> > > be set. Or am I missing something?
> > 
> > So, why do all my iMX6 platforms now randomly spit out:
> > 
> > "imx-sdma 20ec000.sdma: Timeout waiting for CH0 ready"
> 
> This is due to a DT misconfiguration which was uncovered with a recent
> driver change (25aaa75df1e6 dmaengine: imx-sdma: add clock ratio 1:1
> check) and fixed with (941acd566b18 dmaengine: imx-sdma: Only check
> ratio on parts that support 1:1). Please switch to a recent stable
> kernel, 5.1.5 has the fix included.

Please point to the fix, thanks.

-- 
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