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

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Date:   Tue, 3 Dec 2019 14:09:22 -0800
From:   Ray Jui <ray.jui@...adcom.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Andrew Murray <andrew.murray@....com>
Cc:     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 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.

>>> +             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);
>