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

[Lucas Stach <l.stach@pengutronix.de>]

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.

Regards,
Lucas