Re: [PATCH v2 1/3] platform/x86: intel_scu_ipc: Check status after timeout in busy_loop()

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Thu, Sep 07, 2023 at 01:11:17PM -0700, Stephen Boyd wrote:
> Quoting Mika Westerberg (2023-09-06 22:35:13)
> > On Wed, Sep 06, 2023 at 11:09:41AM -0700, Stephen Boyd wrote:
> > > It's possible for the polling loop in busy_loop() to get scheduled away
> > > for a long time.
> > >
> > >   status = ipc_read_status(scu); // status = IPC_STATUS_BUSY
> > >   <long time scheduled away>
> > >   if (!(status & IPC_STATUS_BUSY))
> > >
> > > If this happens, then the status bit could change while the task is
> > > scheduled away and this function would never read the status again after
> > > timing out. Instead, the function will return -ETIMEDOUT when it's
> > > possible that scheduling didn't work out and the status bit was cleared.
> > > Bit polling code should always check the bit being polled one more time
> > > after the timeout in case this happens.
> > >
> > > Fix this by reading the status once more after the while loop breaks.
> > >
> > > Cc: Prashant Malani <pmalani@chromium.org>
> > > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > > Fixes: e7b7ab3847c9 ("platform/x86: intel_scu_ipc: Sleeping is fine when polling")
> > > Signed-off-by: Stephen Boyd <swboyd@chromium.org>
> > > ---
> > >
> > > This is sufficiently busy so I didn't add any tags from previous round.
> > >
> > >  drivers/platform/x86/intel_scu_ipc.c | 11 +++++++----
> > >  1 file changed, 7 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/platform/x86/intel_scu_ipc.c b/drivers/platform/x86/intel_scu_ipc.c
> > > index 6851d10d6582..b2a2de22b8ff 100644
> > > --- a/drivers/platform/x86/intel_scu_ipc.c
> > > +++ b/drivers/platform/x86/intel_scu_ipc.c
> > > @@ -232,18 +232,21 @@ static inline u32 ipc_data_readl(struct intel_scu_ipc_dev *scu, u32 offset)
> > >  static inline int busy_loop(struct intel_scu_ipc_dev *scu)
> > >  {
> > >       unsigned long end = jiffies + IPC_TIMEOUT;
> > > +     u32 status;
> > >
> > >       do {
> > > -             u32 status;
> > > -
> > >               status = ipc_read_status(scu);
> > >               if (!(status & IPC_STATUS_BUSY))
> > > -                     return (status & IPC_STATUS_ERR) ? -EIO : 0;
> > > +                     goto not_busy;
> > >
> > >               usleep_range(50, 100);
> > >       } while (time_before(jiffies, end));
> > >
> > > -     return -ETIMEDOUT;
> > > +     status = ipc_read_status(scu);
> >
> > Does the issue happen again if we get scheduled away here for a long
> > time? ;-)
> 
> Given the smiley I'll assume you're making a joke. But to clarify, the
> issue can't happen again because we've already waited at least
> IPC_TIMEOUT jiffies, maybe quite a bit more, so if we get scheduled away
> again it's a non-issue. If the status is still busy here then it's a
> timeout guaranteed.

Got it thanks!

> > Regardless, I'm fine with this as is but if you make any changes, I
> > would prefer see readl_busy_timeout() used here instead (as was in the
> > previous version).
> 
> We can't use readl_busy_timeout() (you mean readl_poll_timeout() right?)
> because that implements the timeout with timekeeping and we don't know
> if this is called from suspend paths after timekeeping is suspended or
> from early boot paths where timekeeping isn't started.

Yes readl_poll_timeout(). :)

I don't think this code is used anymore outside of regular paths. It
used to be with the Moorestown/Medfield board support code but that's
gone already. Grepping for the users also don't reveal anything that
could be using it early at boot.