Re: [PATCH 05/11] mmc: core: Clarify sdio_irq_pending flag for MMC_CAP2_SDIO_IRQ_NOTHREAD

[Doug Anderson <dianders@chromium.org>]

Hi,

On Fri, Sep 6, 2019 at 2:20 AM Ulf Hansson <ulf.hansson@linaro.org> wrote:
>
> On Fri, 6 Sep 2019 at 01:47, Doug Anderson <dianders@chromium.org> wrote:
> >
> > Hi,
> >
> > On Tue, Sep 3, 2019 at 7:22 AM Ulf Hansson <ulf.hansson@linaro.org> wrote:
> > >
> > > In the single SDIO IRQ handler case, the sdio_irq_pending flag is used to
> > > avoid reading the SDIO_CCCR_INTx register and instead immediately call the
> > > SDIO func's >irq_handler() callback.
> > >
> > > To clarify the use behind the flag for the MMC_CAP2_SDIO_IRQ_NOTHREAD case,
> > > let's set the flag from inside sdio_signal_irq(), rather from
> > > sdio_run_irqs().
> >
> > I'm having a hard time parsing the above statement...  Can you reword
> > and maybe I'll understand?
>
> Sure, I admit, it's not very good. :-) How about the below.
>
> The sdio_irq_pending flag is used to let host drivers indicate that it
> has signaled an IRQ. If that is the case and we only have a single
> SDIO func that have claimed an SDIO IRQ, our assumption is that we can
> avoid reading the SDIO_CCCR_INTx register and just call the SDIO func
> irq handler immediately. This makes sense, but the flag is set/cleared
> in a somewhat messy order, let's fix that up according to below.
>
> First, the flag is currently set in sdio_run_irqs(), which is executed
> as a work that was scheduled from sdio_signal_irq(). To make it more
> implicit that the host have signaled an IRQ, let's instead immediately
> set the flag in sdio_signal_irq(). This also makes the behavior
> consistent with host drivers that uses the legacy,
> mmc_signal_sdio_irq() API. This have no functional impact, because we
> don't expect host drivers to call sdio_signal_irq() until after the
> work (sdio_run_irqs()) have been executed anyways.
>
> Second, currently we never clears the flag when using the
> sdio_run_irqs() work, but only when using the sdio_irq_thread(). Let
> make the behavior consistent, by moving the flag to be cleared inside
> the common process_sdio_pending_irqs() function. Additionally, tweak
> the behavior of the flag slightly, by avoiding to clear it unless we
> processed the SDIO IRQ. The purpose with this at this point, is to
> keep the information about whether there have been an SDIO IRQ
> signaled by the host, so at system resume we can decide to process it
> without reading the SDIO_CCCR_INTx register.
>
> >
> >
> > > Moreover, let's also reset the flag when the SDIO IRQ have
> > > been properly processed.
> > >
> > > Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
> > > ---
> > >  drivers/mmc/core/sdio_irq.c | 9 ++++++---
> > >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > Nice!  This looks like it addresses some of the things that came up in
> > the previous discussion [1] and should be a nice improvement.  From
> > re-reading that discussion that will probably change the behvaior
> > slightly (hopefully for the better) in the single-function case where
> > we might actually poll CCCR_INTx sometimes now.
>
> Correct!
>
> >
> >
> > > diff --git a/drivers/mmc/core/sdio_irq.c b/drivers/mmc/core/sdio_irq.c
> > > index f75043266984..0962a4357d54 100644
> > > --- a/drivers/mmc/core/sdio_irq.c
> > > +++ b/drivers/mmc/core/sdio_irq.c
> > > @@ -59,6 +59,7 @@ static int process_sdio_pending_irqs(struct mmc_host *host)
> > >  {
> > >         struct mmc_card *card = host->card;
> > >         int i, ret, count;
> > > +       bool sdio_irq_pending = host->sdio_irq_pending;
> > >         unsigned char pending;
> > >         struct sdio_func *func;
> > >
> > > @@ -66,13 +67,16 @@ static int process_sdio_pending_irqs(struct mmc_host *host)
> > >         if (mmc_card_suspended(card))
> > >                 return 0;
> > >
> > > +       /* Clear the flag to indicate that we have processed the IRQ. */
> > > +       host->sdio_irq_pending = false;
> > > +
> > >         /*
> > >          * Optimization, if there is only 1 function interrupt registered
> > >          * and we know an IRQ was signaled then call irq handler directly.
> > >          * Otherwise do the full probe.
> > >          */
> > >         func = card->sdio_single_irq;
> > > -       if (func && host->sdio_irq_pending) {
> > > +       if (func && sdio_irq_pending) {
> > >                 func->irq_handler(func);
> > >                 return 1;
> > >         }
> > > @@ -110,7 +114,6 @@ static void sdio_run_irqs(struct mmc_host *host)
> > >  {
> > >         mmc_claim_host(host);
> > >         if (host->sdio_irqs) {
> > > -               host->sdio_irq_pending = true;
> > >                 process_sdio_pending_irqs(host);
> > >                 if (host->ops->ack_sdio_irq)
> > >                         host->ops->ack_sdio_irq(host);
> > > @@ -128,6 +131,7 @@ void sdio_irq_work(struct work_struct *work)
> > >
> > >  void sdio_signal_irq(struct mmc_host *host)
> > >  {
> > > +       host->sdio_irq_pending = true;
> >
> > Is this safe to do without claiming the host or any other type of
> > locking?  sdio_signal_irq() is called directly from the interrupt
> > handler on dw_mmc with no locks held at all.  Could we have races /
> > problems with weakly ordered memory?
>
> At this point, for $subject patch and @subject series, I don't see any
> issues. But perhaps when we go forward and start using the flag
> slightly differently.

Lockless concurrency always makes my head hurt (especially when I try
to consider weakly ordered memory) and I've learned that the only way
I can reason about it and have any belief that I got it right is to
always make sure I access values in a context where things are locked.
:-P

Let's see if I can figure out any actual problem, though...

Certainly the queue_delayed_work() would act as a barrier so you don't
have to worry about the worker not seeing the "= true".

I suppose it's definitely possible that (if the worker is already
running) that our "= true" will get clobbered by an "= false" from a
previous instance of the worker running.  I guess that's unlikely
because we can't get a second IRQ signaled until the "->ack_sdio_irq"
ran and presumably there's enough stuff after the "= false" that one
of them would have a barrier that made sure that the "= false" didn't
affect us in a delayed way.

So I guess we're fine...

-Doug