Re: [PATCH V12 4/6] PCI: loongson: Improve the MRRS quirk for LS7A

[Rob Herring <robh@kernel.org>]

On Sat, Feb 26, 2022 at 06:47:29PM +0800, Huacai Chen wrote:
> In new revision of LS7A, some PCIe ports support larger value than 256,
> but their maximum supported MRRS values are not detectable. Moreover,
> the current loongson_mrrs_quirk() cannot avoid devices increasing its
> MRRS after pci_enable_device(), and some devices (e.g. Realtek 8169)
> will actually set a big value in its driver. So the only possible way
> is configure MRRS of all devices in BIOS, and add a pci host bridge bit
> flag (i.e., no_inc_mrrs) to stop the increasing MRRS operations.
> 
> However, according to PCIe Spec, it is legal for an OS to program any
> value for MRRS, and it is also legal for an endpoint to generate a Read
> Request with any size up to its MRRS. As the hardware engineers say, the
> root cause here is LS7A doesn't break up large read requests. In detail,
> LS7A PCIe port reports CA (Completer Abort) if it receives a Memory Read
> request with a size that's "too big" ("too big" means larger than the
> PCIe ports can handle, which means 256 for some ports and 4096 for the
> others, and of course this is a problem in the LS7A's hardware design).
> 
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
>  drivers/pci/controller/pci-loongson.c | 47 ++++++++++-----------------
>  drivers/pci/pci.c                     |  6 ++++
>  include/linux/pci.h                   |  1 +
>  3 files changed, 25 insertions(+), 29 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
> index 48d9d283cb59..ba182f9d5718 100644
> --- a/drivers/pci/controller/pci-loongson.c
> +++ b/drivers/pci/controller/pci-loongson.c
> @@ -67,37 +67,26 @@ DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
>  DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
>  			DEV_LS7A_LPC, system_bus_quirk);
>  
> -static void loongson_mrrs_quirk(struct pci_dev *dev)
> +static void loongson_mrrs_quirk(struct pci_dev *pdev)
>  {
> -	struct pci_bus *bus = dev->bus;
> -	struct pci_dev *bridge;
> -	static const struct pci_device_id bridge_devids[] = {
> -		{ PCI_VDEVICE(LOONGSON, DEV_PCIE_PORT_0) },
> -		{ PCI_VDEVICE(LOONGSON, DEV_PCIE_PORT_1) },
> -		{ PCI_VDEVICE(LOONGSON, DEV_PCIE_PORT_2) },
> -		{ 0, },
> -	};
> -
> -	/* look for the matching bridge */
> -	while (!pci_is_root_bus(bus)) {
> -		bridge = bus->self;
> -		bus = bus->parent;
> -		/*
> -		 * Some Loongson PCIe ports have a h/w limitation of
> -		 * 256 bytes maximum read request size. They can't handle
> -		 * anything larger than this. So force this limit on
> -		 * any devices attached under these ports.
> -		 */
> -		if (pci_match_id(bridge_devids, bridge)) {
> -			if (pcie_get_readrq(dev) > 256) {
> -				pci_info(dev, "limiting MRRS to 256\n");
> -				pcie_set_readrq(dev, 256);
> -			}
> -			break;
> -		}
> -	}
> +	/*
> +	 * Some Loongson PCIe ports have h/w limitations of maximum read
> +	 * request size. They can't handle anything larger than this. So
> +	 * force this limit on any devices attached under these ports.
> +	 */
> +	struct pci_host_bridge *bridge = pci_find_host_bridge(pdev->bus);
> +
> +	if (!bridge)

Isn't this condition impossible? I'd drop and just let the NULL 
dereference fault happen rather than continue on silently.

> +		return;
> +
> +	bridge->no_inc_mrrs = 1;
>  }
> -DECLARE_PCI_FIXUP_ENABLE(PCI_ANY_ID, PCI_ANY_ID, loongson_mrrs_quirk);
> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
> +			DEV_PCIE_PORT_0, loongson_mrrs_quirk);
> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
> +			DEV_PCIE_PORT_1, loongson_mrrs_quirk);
> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
> +			DEV_PCIE_PORT_2, loongson_mrrs_quirk);
>  
>  static struct loongson_pci *pci_bus_to_loongson_pci(struct pci_bus *bus)
>  {
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 9ecce435fb3f..72a15bf9eee8 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5993,6 +5993,7 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
>  {
>  	u16 v;
>  	int ret;
> +	struct pci_host_bridge *bridge = pci_find_host_bridge(dev->bus);
>  
>  	if (rq < 128 || rq > 4096 || !is_power_of_2(rq))
>  		return -EINVAL;
> @@ -6011,6 +6012,11 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
>  
>  	v = (ffs(rq) - 8) << 12;
>  
> +	if (bridge->no_inc_mrrs) {
> +		if (rq > pcie_get_readrq(dev))
> +			return -EINVAL;
> +	}
> +
>  	ret = pcie_capability_clear_and_set_word(dev, PCI_EXP_DEVCTL,
>  						  PCI_EXP_DEVCTL_READRQ, v);
>  
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 8253a5413d7c..01a464eb640a 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -569,6 +569,7 @@ struct pci_host_bridge {
>  	void		*release_data;
>  	unsigned int	ignore_reset_delay:1;	/* For entire hierarchy */
>  	unsigned int	no_ext_tags:1;		/* No Extended Tags */
> +	unsigned int	no_inc_mrrs:1;		/* No Increase MRRS */
>  	unsigned int	native_aer:1;		/* OS may use PCIe AER */
>  	unsigned int	native_pcie_hotplug:1;	/* OS may use PCIe hotplug */
>  	unsigned int	native_shpc_hotplug:1;	/* OS may use SHPC hotplug */
> -- 
> 2.27.0
>