linux-kernel - Re: [PATCH v3 2/6] PCI: iproc: Add INTx support with better modeling

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Message-ID: <CAHp75VfyKAg4OhzUa4swGXOGTvJ5fVO8mhGSG=5HAUP__M-URQ@mail.gmail.com>
Date:   Wed, 4 Dec 2019 10:29:51 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Ray Jui <ray.jui@...adcom.com>
Cc:     Andrew Murray <andrew.murray@....com>,
        Srinath Mannam <srinath.mannam@...adcom.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        linux-pci@...r.kernel.org, devicetree <devicetree@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/6] PCI: iproc: Add INTx support with better modeling

On Wed, Dec 4, 2019 at 12:09 AM Ray Jui <ray.jui@...adcom.com> wrote:
> On 12/3/19 11:27 AM, Andy Shevchenko wrote:
> > On Tue, Dec 3, 2019 at 5:55 PM Andrew Murray <andrew.murray@....com> wrote:
> >> On Tue, Dec 03, 2019 at 10:27:02AM +0530, Srinath Mannam wrote:
> >
> >>> +     /* go through INTx A, B, C, D until all interrupts are handled */
> >>> +     do {
> >>> +             status = iproc_pcie_read_reg(pcie, IPROC_PCIE_INTX_CSR);
> >>
> >> By performing this read once and outside of the do/while loop you may improve
> >> performance. I wonder how probable it is to get another INTx whilst handling
> >> one?
> >
> > May I ask how it can be improved?
> > One read will be needed any way, and so does this code.
> >
>
> I guess the current code will cause the IPROC_PCIE_INTX_CSR register to
> be read TWICE, if it's ever set to start with.
>
> But then if we do it outside of the while loop, if we ever receive an
> interrupt while servicing one, the interrupt will still need to be
> serviced, and in this case, it will cause additional context switch
> overhead by going out and back in the interrupt context.
>
> My take is that it's probably more ideal to leave this portion of code
> as it is.

Can't we simple drop a do-while completely and leave only
for_each_set_bit() loop?

>
> >>> +             for_each_set_bit(bit, &status, PCI_NUM_INTX) {
> >>> +                     virq = irq_find_mapping(pcie->irq_domain, bit);
> >>> +                     if (virq)
> >>> +                             generic_handle_irq(virq);
> >>> +                     else
> >>> +                             dev_err(dev, "unexpected INTx%u\n", bit);
> >>> +             }
> >>> +     } while ((status & SYS_RC_INTX_MASK) != 0);
> >



-- 
With Best Regards,
Andy Shevchenko