linux-kernel - Re: A question about shpcp.

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Date:   Tue, 2 Jul 2019 14:21:17 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mingfangsen <mingfangsen@...wei.com>
Subject: Re: A question about shpcp.

On Thu, Jun 27, 2019 at 12:16:18PM +0000, linmiaohe wrote:
> In qemu+shpcp+pcie scene, hotplug a network card(virtio) would take more
> than 5 seconds. It's because 5 seconds delayed_work in func
> handle_button_press_event with case STATIC_STATE. And this will break some
> protocols with timeout within 5 seconds.
> It's very nice of you if you could tell me why is 5*HZ there and if this
> delay can be reduced?

According to Table 2-4 of the PCI Standard Hot-Plug Controller and
Subsystem Specification, "System software is waiting 5 seconds to
provide the user with an opportunity to cancel the hot-plug operation."
(http://drydkim.com/MyDocuments/PCI%20Spec/specifications/shpc1_0.pdf)

So the reason the delay can't be reduced or removed is because the spec
mandates it.

Thanks,

Lukas