linux-kernel - Re: [PATCH v3 8/9] pci: Tune secondary bus reset timing

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Message-ID: <51FAD338.5000306@redhat.com>
Date:	Thu, 01 Aug 2013 17:29:28 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
CC:	bhelgaas@...gle.com, linux-pci@...r.kernel.org,
	indou.takao@...fujitsu.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 8/9] pci: Tune secondary bus reset timing

On 08/01/2013 12:55 PM, Alex Williamson wrote:
> The PCI spec indicates that with stable power, reset needs to be
> asserted for a minimum of 1ms (Trst).  Seems like we should be able
> to assume power is stable for a runtime secondary bus reset.  The
> current code has always used 100ms with no explanation where that
> came from.  The aer_do_secondary_bus_reset() function uses 2ms, but
> that seems to be a misinterpretation of the PCIe spec, where hot
> reset is implemented by TS1 ordered sets containing the hot reset
> command.  After a 2ms delay the state machine enters the detect state,
> but to generate a link down, only two consecutive TS1 hot reset
> ordered sets are requred.  1ms should be plenty for that.
>
> After reset is de-asserted we must wait for devices to complete
> initialization.  The specs refer to this as "recovery time" (Trhfa).
> For PCI this is 2^25 clock cycles or 2^26 for PCI-X.  For minimum
> bus speeds, both of those come to 1s.  PCIe "softens" this
> requirement with the Configuration Request Retry Status (CRS)
> completion status.  Theoretically we could use CRS to shorten the
> wait time.  We don't make use of that here, using a fixed 1s delay
> to allow devices to re-initialize.
Unfortunately, I don't think CRS is widely supported to make it worth
the additional checking & use, atm.

>
> Signed-off-by: Alex Williamson<alex.williamson@...hat.com>
> ---
>   drivers/pci/pci.c |   15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 3e71887..a5c6a9b 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3291,11 +3291,22 @@ void pci_reset_bridge_secondary_bus(struct pci_dev *dev)
>   	pci_read_config_word(dev, PCI_BRIDGE_CONTROL,&ctrl);
>   	ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
>   	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> -	msleep(100);
> +	/*
> +	 * PCI spec v3.0 7.6.4.2 requires minimum Trst of 1ms.
> +	 */
> +	msleep(1);
>
>   	ctrl&= ~PCI_BRIDGE_CTL_BUS_RESET;
>   	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl);
> -	msleep(100);
> +
> +	/*
> +	 * Trhfa for conventional PCI is 2^25 clock cycles.
> +	 * Assuming a minimum 33MHz clock this results in a 1s
> +	 * delay before we can consider subordinate devices to
> +	 * be re-initialized.  PCIe has some ways to shorten this,
> +	 * but we don't make use of them yet.
> +	 */
> +	ssleep(1);
Can't bus speed be determined from (config space) status bits, so
this time can be minimized, esp. on modern, PCIe busses/links ?
Not too many 33Mhz, legacy PCI busses that this type of
timing is desired, or will be done to (for device assignment/vfio). :-/

>   }
>   EXPORT_SYMBOL_GPL(pci_reset_bridge_secondary_bus);
>
>
> --
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