linux-kernel - Re: [PATCH v2] PCI: keystone: add a pci quirk to limit mrrs

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Message-ID: <20140905192329.GE8080@google.com>
Date:	Fri, 5 Sep 2014 13:23:29 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Murali Karicheri <m-karicheri2@...com>
Cc:	linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, jgunthorpe@...idianresearch.com
Subject: Re: [PATCH v2] PCI: keystone: add a pci quirk to limit mrrs

On Fri, Aug 08, 2014 at 03:45:32PM -0400, Murali Karicheri wrote:
> Keystone PCI controller has a limitation that memory read request
> size must not exceed 256 bytes. This is a hardware limitation and
> add a quirk to force this limit on all downstream devices by
> updating mrrs.
> 
> Signed-off-by: Murali Karicheri <m-karicheri2@...com>
> ---
>  -v2: made the quirk happens after tuning
>  -v1: changed printk to indicate PCI bdf
>  This applies on top of the Keystone PCI controller patch series
>  at http://thread.gmane.org/gmane.linux.kernel.pci/33523
>  drivers/pci/host/pci-keystone.c |   40 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/drivers/pci/host/pci-keystone.c b/drivers/pci/host/pci-keystone.c
> index c1cfaef..a132622 100644
> --- a/drivers/pci/host/pci-keystone.c
> +++ b/drivers/pci/host/pci-keystone.c
> @@ -42,8 +42,48 @@
>  /* DEV_STAT_CTRL */
>  #define PCIE_CAP_BASE		0x70
>  
> +/* PCIE controller device IDs */
> +#define PCIE_RC_K2HK		0xb008
> +#define PCIE_RC_K2E		0xb009
> +#define PCIE_RC_K2L		0xb00a

A Root Complex doesn't appear in config space as a PCI device, so I think
these are actually Root Port devices, aren't they?

I suppose the spec would allow Root Complex Integrated Endpoints, on which
MRRS could also be set, but I think your quirk ignores them because they
would appear on the root bus.  Is ignoring them OK, or would you want to
set MRRS for those as well?

> +
>  #define to_keystone_pcie(x)	container_of(x, struct keystone_pcie, pp)
>  
> +static void quirk_limit_mrrs(struct pci_dev *dev)
> +{
> +	struct pci_bus *bus = dev->bus;
> +	struct pci_dev *bridge = bus->self;
> +
> +	if (pci_is_root_bus(bus))
> +		return;
> +
> +	/* look for the host bridge */
> +	while (!pci_is_root_bus(bus)) {
> +		bridge = bus->self;
> +		bus = bus->parent;
> +	}
> +
> +	if (bridge) {
> +		u16 id;
> +
> +		/*
> +		 * Keystone PCI controller has a h/w limitation of
> +		 * 256 bytes maximum read request size. It can't handle
> +		 * anything higher than this. So force this limit on
> +		 * all downstream devices
> +		 */
> +		pci_read_config_word(bridge, PCI_DEVICE_ID, &id);

I think you should check the Vendor ID here as well.  Otherwise, there's a
possibility of applying this quirk to devices under a bridge with the same
Device ID but different Vendor ID.

And I think you should be able to use the bridge->vendor and bridge->device
fields instead of reading them from config space.  Actually, this might be
a good place to use pci_match_id().

> +		if ((id == PCIE_RC_K2HK) || (id == PCIE_RC_K2E) ||
> +		    (id == PCIE_RC_K2L)) {
> +			if (pcie_get_readrq(dev) > 256) {
> +				dev_info(&dev->dev, "limiting mrrs to 256\n");
> +				pcie_set_readrq(dev, 256);
> +			}
> +		}
> +	}
> +}
> +DECLARE_PCI_FIXUP_ENABLE(PCI_ANY_ID, PCI_ANY_ID, quirk_limit_mrrs);
> +
>  static int ks_pcie_establish_link(struct keystone_pcie *ks_pcie)
>  {
>  	struct pcie_port *pp = &ks_pcie->pp;
> -- 
> 1.7.9.5
> 
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