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

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On 12/3/19 11:27 AM, Andy Shevchenko wrote:
On Tue, Dec 3, 2019 at 5:55 PM Andrew Murray <andrew.murray@xxxxxxx> 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);




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