Messages in this thread | | | Subject | Re: [PATCH v3 2/6] PCI: iproc: Add INTx support with better modeling | From | Ray Jui <> | Date | Wed, 4 Dec 2019 10:36:31 -0800 |
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On 12/4/19 8:07 AM, Andrew Murray wrote: > On Wed, Dec 04, 2019 at 10:29:51AM +0200, Andy Shevchenko wrote: >> On Wed, Dec 4, 2019 at 12:09 AM Ray Jui <ray.jui@broadcom.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@arm.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. > > Yes it's a trade off - if you dropped the do/while loop and thus had a single > read you'd reduce the overhead on interrupt handling in every case except > where another INTx is received whilst in this function. But as you point out > each time that does happen you'll pay the penalty of a context switch. >
Exactly, it's a tradeoff between: 1) saving one register read (which is likely in the 10th of nanosecond range) in all INTx handling; and 2) saving context switches (which is likely in 10th of microsecond range) in cases when we have multiple INTx when servicing it.
The current implementation takes 2), which I thought it makes sense.
> I don't have any knowledge of this platform so I have no idea if such a change > would be good/bad or material. However I thought I'd point it out. Looking at > the other controller drivers, some handle in a loop and some don't. > > >>> >>> 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? >>
Like both Andrew and I pointed out. There's a tradeoff here. Could you please help to justify why you favor 1) than 2)?
> > I'm happy either way. > > Thanks, > > Andrew Murray > >>> >>>>>> + 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
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