linux-kernel - Re: [PATCH 2/3] PCI: pciehp: Wait for PDS if in-band presence is disabled

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Date:   Thu, 10 Oct 2019 08:37:02 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Stuart Hayes <stuart.w.hayes@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Austin Bolen <austin_bolen@...l.com>,
        Keith Busch <keith.busch@...el.com>,
        Alexandru Gagniuc <mr.nuke.me@...il.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Sinan Kaya <okaya@...nel.org>,
        Oza Pawandeep <poza@...eaurora.org>, linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>
Subject: Re: [PATCH 2/3] PCI: pciehp: Wait for PDS if in-band presence is disabled

On Wed, Oct 9, 2019 at 11:05 PM Stuart Hayes <stuart.w.hayes@...il.com> wrote:
>
> From: Alexandru Gagniuc <mr.nuke.me@...il.com>
>
> When inband presence is disabled, PDS may come up at any time, or not
> at all. PDS being low may indicate that the card is still mating, and
> we could expect contact bounce to bring down the link as well.
>
> It is reasonable to assume that most cards will mate in a hotplug slot
> in about a second. Thus, when we know PDS only reflects out-of-band
> presence, it's worthwhile to wait the extra second or so to make sure
> the card is properly mated before loading the driver, and to prevent
> the hotplug code from disabling a device if the presence detect change
> goes active after the device is enabled.

> +static void pcie_wait_for_presence(struct pci_dev *pdev)
> +{
> +       int timeout = 1250;
> +       bool pds;
> +       u16 slot_status;
> +
> +       while (true) {
> +               pcie_capability_read_word(pdev, PCI_EXP_SLTSTA, &slot_status);
> +               pds = !!(slot_status & PCI_EXP_SLTSTA_PDS);
> +               if (pds || timeout <= 0)
> +                       break;
> +               msleep(10);
> +               timeout -= 10;
> +       }

Can we avoid infinite loops? They are hard to parse (in most cases,
and especially when it's a timeout loop)

unsigned int retries = 125; // 1250 ms

do {
 ...
} while (--retries);

> +
> +       if (!pds)
> +               pci_info(pdev, "Presence Detect state not set in 1250 msec\n");
> +}

-- 
With Best Regards,
Andy Shevchenko