Re: [PATCH 2/3] mtd: mediatek: driver for MTK Smart Device Gen1 NAND

Brian Norris
2016-03-08T20:21:00Z

On Tue, Mar 08, 2016 at 03:08:33PM -0500, Jorge Ramirez-Ortiz wrote:
> On 03/08/2016 01:17 PM, Brian Norris wrote:
> >> > You may want to use readl_relaxed_poll_timeout() (even though there's
> >> > no way to specify a range).
> >> > This comment applies to all the places where you're implementing this
> >> > kind of loop.
> > What's more, this timeout loop (and probably many of the others) is
> > wrong. You need to do one last status check before declaring a timeout,
> > since the device may become ready while you're sleeping. It's the same
> > problem as we've resolved here:
> >
> > http://git.infradead.org/l2-mtd.git/commitdiff/9ebfdf5b18493f338237ef9861a555c2f79b0c17
> > Subject: "mtd: nand: check status before reporting timeout"
>
> I don't think it is quite the same scenario: in the case that you are describing
> the wait is actually rescheduling and yes, that could kick the process out of
> the CPU for a while (in the millisecond range).
>
> In this driver however, we are either sleeping for a bounded amount of time (+/-
> a margin) in microseconds OR calling cpu_relax() which is just a memory barrier
> in arm.
> In the former case, I agree that sleeping for a microsecond range (since there
> is not a guaranteed maximum jitter in theory) could go wild but that is highly
> unlikely.

Right, it's not exactly the same, but it is the same in concept. It's
irrelevant whether the time is bounded or not.

> If you feel strongly about it I don't mind adding an additional check after any
> form of sleep (not so sure about adding it after a cpu_relax) but I don't think
> it is needed.

It is non-negotiable that your timeout loops must be logically correct.
That is, you must recheck the exit condition before you declare a
timeout.

If you just follow Boris's suggestion of using the helper macros, then
you'll be fine.

Brian