linux-kernel - Re: Workaround for Intel MPS errata

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Date:	Thu, 29 Sep 2011 19:21:58 -0700
From:	Jesse Brandeburg <jesse.brandeburg@...il.com>
To:	Jon Mason <mason@...i.com>
Cc:	Avi Kivity <avi@...hat.com>, Sven Schnelle <svens@...ckframe.org>,
	Simon Kirby <sim@...tway.ca>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Niels Ole Salscheider <niels_ole@...scheider-online.de>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: Workaround for Intel MPS errata

On Thu, Sep 29, 2011 at 5:16 PM, Jon Mason <mason@...i.com> wrote:
> Hey Avi,
> Can you try this patch?  It should resolve the issue you are seeing.
>
> Thanks,
> Jon
>
>    PCI: Workaround for Intel MPS errata
>
>    Intel 5000 and 5100 series memory controllers have a known issue if read
>    completion coalescing is enabled (the default setting) and the PCI-E
>    Maximum Payload Size is set to 256B.  To work around this issue, disable
>    read completion coalescing if the MPS is 256B.

Hey Jon, glad I could help out by pointing this erratum out on IRC
today.  The patch looks mostly fine, with one nit, see below.


> +       /* Disable read completion coalescing to allow an MPS of 256 */
> +       if (mps == 256) {
> +               int err;
> +               u16 rcc;
> +
> +               /* Intel errata specifies bits to change but does not say what
> +                * they are.  Keeping them magical until such time as the
> +                * registers and values can be explained.
> +                */
> +               err = pci_read_config_word(dev, 0x48, &rcc);
> +               if (err) {
> +                       dev_err(&dev->dev, "Error attempting to read the read "
> +                               "completion coalescing register.\n");
> +                       return;
> +               }
> +
> +               rcc &= ~(1 << 10);
> +
> +               err = pci_write_config_word(dev, 0x48, rcc);
> +               if (err) {
> +                       dev_err(&dev->dev, "Error attempting to read the read "

this should be "to write the read "

> +                               "completion coalescing register.\n");
> +                       return;
> +               }

do you have to do anything to change the bit back to 1 if something
sets the mps back to 128?
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