linux-kernel - Re: [PATCH 2/2] pinctrl/amd: use byte access to clear irq/wake status bits

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Date:   Fri, 20 Jul 2018 17:38:17 -0600
From:   Daniel Kurtz <djkurtz@...omium.org>
To:     Daniel Drake <drake@...lessm.com>
Cc:     Shyam-sundar.S-k@....com, Nehal-bakulchandra.Shah@....com,
        Ken.Xue@....com, Thomas Gleixner <tglx@...utronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        "open list:PIN CONTROL SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] pinctrl/amd: use byte access to clear irq/wake status bits

Hi Daniel,

On Tue, Jul 17, 2018 at 6:30 AM Daniel Drake <drake@...lessm.com> wrote:
>
> On Mon, Jul 16, 2018 at 7:57 PM, Daniel Kurtz <djkurtz@...omium.org> wrote:
> > Commit 6afb10267c1692 ("pinctrl/amd: fix masking of GPIO interrupts")
> > changed to the clearing of interrupt status bits to a RMW in a critical
> > section.  This works, but is a bit overkill.
> >
> > The relevant interrupt/wake status bits are in the Most Significant Byte
> > of a 32-bit word.  These two are the only write-able bits in this byte.
>
> I don't have the hardware to test this any more, and I also don't have
> any docs to double if those are really the only writable bits, but
> looking at the existing driver code it does seem to be the case.
>
> I think you should retain the comment noting that the value of the
> register may have changed since it was read just a few lines above
> (and hence explaining more precisely why we make the special effort
> just to modify the MSB), just in case there is further rework of this
> code in future and we end up walking into the same trap. It was one of
> those issues that took a frustratingly long time to figure out...

Sounds reasonable.  How about:

-                       /* Clear interrupt.
-                        * We must read the pin register again, in case the
-                        * value was changed while executing
-                        * generic_handle_irq() above.
+                       /*
+                        * Write-1-to-clear irq/wake status bits in MSByte.
+                        * All other bits in this byte are read-only.
+                        * This avoids modifying the lower 24-bits
because they may have
+                        *  changed while executing generic_handle_irq() above.
                         */


>
> Thanks
> Daniel