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

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Message-ID: <540A1D54.9030103@ti.com>
Date:	Fri, 5 Sep 2014 16:30:12 -0400
From:	Murali Karicheri <m-karicheri2@...com>
To:	Bjorn Helgaas <bhelgaas@...gle.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 09/05/2014 03:23 PM, Bjorn Helgaas wrote:
> 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?

Yes.

>
> 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?

How does the Root Complex integrated endpoints are designed? Our PCI can 
act as a RC or EP and not both at the same time. So I guess this is not 
applicable.

>
>> +
>>   #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().

Ok.

>
>> +		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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