Message-ID: <20170608132620.17fc7c96@bbrezillon>
Date:   Thu, 8 Jun 2017 13:26:20 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Dinh Nguyen <dinguyen@...nel.org>,
        Enrico Jorns <ejo@...gutronix.de>,
        Richard Weinberger <richard@....at>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Vasut <marek.vasut@...il.com>,
        linux-mtd@...ts.infradead.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Chuanxiao Dong <chuanxiao.dong@...el.com>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v5 10/23] mtd: nand: denali: rework interrupt handling

On Thu, 8 Jun 2017 19:41:39 +0900
Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:

> Hi Boris,
> 
> 
> 2017-06-08 16:12 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
> > Le Thu, 8 Jun 2017 15:10:18 +0900,
> > Masahiro Yamada <yamada.masahiro@...ionext.com> a écrit :
> >  
> >> Hi Boris,
> >>
> >>
> >> 2017-06-07 22:57 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:  
> >> > On Wed,  7 Jun 2017 20:52:19 +0900
> >> > Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:
> >> >
> >> >  
> >> >> -/*
> >> >> - * This is the interrupt service routine. It handles all interrupts
> >> >> - * sent to this device. Note that on CE4100, this is a shared interrupt.
> >> >> - */
> >> >> -static irqreturn_t denali_isr(int irq, void *dev_id)
> >> >> +static uint32_t denali_wait_for_irq(struct denali_nand_info *denali,
> >> >> +                                 uint32_t irq_mask)
> >> >>  {
> >> >> -     struct denali_nand_info *denali = dev_id;
> >> >> +     unsigned long time_left, flags;
> >> >>       uint32_t irq_status;
> >> >> -     irqreturn_t result = IRQ_NONE;
> >> >>
> >> >> -     spin_lock(&denali->irq_lock);
> >> >> +     spin_lock_irqsave(&denali->irq_lock, flags);
> >> >>
> >> >> -     /* check to see if a valid NAND chip has been selected. */
> >> >> -     if (is_flash_bank_valid(denali->flash_bank)) {
> >> >> -             /*
> >> >> -              * check to see if controller generated the interrupt,
> >> >> -              * since this is a shared interrupt
> >> >> -              */
> >> >> -             irq_status = denali_irq_detected(denali);
> >> >> -             if (irq_status != 0) {
> >> >> -                     /* handle interrupt */
> >> >> -                     /* first acknowledge it */
> >> >> -                     clear_interrupt(denali, irq_status);
> >> >> -                     /*
> >> >> -                      * store the status in the device context for someone
> >> >> -                      * to read
> >> >> -                      */
> >> >> -                     denali->irq_status |= irq_status;
> >> >> -                     /* notify anyone who cares that it happened */
> >> >> -                     complete(&denali->complete);
> >> >> -                     /* tell the OS that we've handled this */
> >> >> -                     result = IRQ_HANDLED;
> >> >> -             }
> >> >> +     irq_status = denali->irq_status;
> >> >> +
> >> >> +     if (irq_mask & irq_status) {
> >> >> +             spin_unlock_irqrestore(&denali->irq_lock, flags);
> >> >> +             return irq_status;
> >> >>       }
> >> >> -     spin_unlock(&denali->irq_lock);
> >> >> -     return result;
> >> >> +
> >> >> +     denali->irq_mask = irq_mask;
> >> >> +     reinit_completion(&denali->complete);  
> >> >
> >> > These 2 instructions should be done before calling
> >> > denali_wait_for_irq() (for example in denali_reset_irq()), otherwise
> >> > you might loose events if they happen between your irq_status read and
> >> > the reinit_completion() call.  
> >>
> >> No.
> >>
> >> denali->irq_lock avoids a race between denali_isr() and
> >> denali_wait_for_irq().
> >>
> >>
> >> The line
> >>      denali->irq_status |= irq_status;
> >> in denali_isr() accumulates all events that have happened
> >> since denali_reset_irq().
> >>
> >> If the interested IRQs have already happened
> >> before denali_wait_for_irq(), it just return immediately
> >> without using completion.
> >>
> >> I do not mind adding a comment like below
> >> if you think my intention is unclear, though.
> >>
> >>         /* Return immediately if interested IRQs have already happend. */
> >>         if (irq_mask & irq_status) {
> >>                 spin_unlock_irqrestore(&denali->irq_lock, flags);
> >>                 return irq_status;
> >>         }
> >>
> >>  
> >
> > My bad, I didn't notice you were releasing the lock after calling
> > reinit_completion(). I still find this solution more complex than my
> > proposal, but I don't care that much.  
> 
> 
> At first, I implemented exactly like you suggested;
>    denali->irq_mask = irq_mask;
>    reinit_completion(&denali->complete)
> in denali_reset_irq().
> 
> 
> IIRC, things were like this.
> 
> Some time later, you memtioned to use ->cmd_ctrl
> instead of ->cmdfunc.
> 
> Then I had a problem when I needed to implement
> denali_check_irq() in
> http://patchwork.ozlabs.org/patch/772395/
> 
> denali_wait_for_irq() is blocked until interested IRQ happens.
> but ->dev_ready() hook should not be blocked.
> It should return if R/B# transition has happened or not.

Nope, it should return whether the NAND is ready or not, not whether a
busy -> ready transition occurred or not. It's typically done by
reading the NAND STATUS register or by checking the R/B pin status.

> So, I accumulate IRQ events in denali->irq_status
> that have happened since denali_reset_irq().

Yep, I see that.

> 
> 
> 
> >>
> >>
> >>  
> >> > You should also clear existing interrupts
> >> > before launching your operation, otherwise you might wakeup on previous
> >> > events.  
> >>
> >>
> >> I do not see a point in your suggestion.
> >>
> >> denali_isr() reads out IRQ_STATUS(i) and immediately clears IRQ bits.
> >>
> >> IRQ events triggered by previous events are accumulated in denali->irq_status.
> >>
> >> denali_reset_irq() clears it.
> >>
> >>         denali->irq_status = 0;  
> >
> > Well, it was just a precaution, in case some interrupts weren't cleared
> > during the previous test (for example if they were masked before the
> > event actually happened, which can occur if you have a timeout, but
> > the event is detected afterward).  
> 
> Turning on/off IRQ mask is problematic.
> So I did not do that.

I don't see why this is a problem. That's how it usually done.

> 
> I enable IRQ mask in driver probe.
> I think this approach is more robust when we consider race conditions
> like you mentioned.

I'd like to hear more about the reasons you think it's more robust
than

* at-probe-time: mask all IRQs and reset IRQ status

* when doing a specific operation:
1/ reset irq status
2/ unmask relevant irqs (based on the operation you're doing)
3/ launch the operation
4/ wait for interrupts
5/ mask irqs and check the wait_for_completion() return code + irq
   status

This approach shouldn't be racy, because you're resetting+unmasking
irqs before starting the real operation (the one supposed to generate
such interrupts). By doing that you also get rid of the extra
->irq_status field, and you don't have to check irq_status before
calling wait_for_completion().

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