linux-kernel - Re: [PATCH V3 2/2] PCI: dwc: Add support to configure for ECRC

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Date:   Mon, 2 Nov 2020 17:02:34 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Vidya Sagar <vidyas@...dia.com>
Cc:     jingoohan1@...il.com, gustavo.pimentel@...opsys.com,
        lorenzo.pieralisi@....com, bhelgaas@...gle.com,
        amurray@...goodpenguin.co.uk, robh@...nel.org, treding@...dia.com,
        jonathanh@...dia.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, kthota@...dia.com,
        mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V3 2/2] PCI: dwc: Add support to configure for ECRC

On Thu, Oct 29, 2020 at 11:09:59AM +0530, Vidya Sagar wrote:
> DesignWare core has a TLP digest (TD) override bit in one of the control
> registers of ATU. This bit also needs to be programmed for proper ECRC
> functionality. This is currently identified as an issue with DesignWare
> IP version 4.90a. This patch does the required programming in ATU upon
> querying the system policy for ECRC.

I guess this is a hardware defect, right?

How much of a problem would it be if we instead added a "no_ecrc"
quirk for this hardware so we never enabled ECRC?

IIUC, the current Linux support of ECRC is a single choice at
boot-time: by default ECRC is not enabled, but if you boot with
"pci=ecrc=on", we turn on ECRC for every device.

That seems like the minimal support, but I think the spec allows ECRC
to be enabled selectively, on individual devices.  I can imagine a
sysfs knob that would allow us to enable/disable ECRC per-device at
run-time.

If we had such a sysfs knob, it would be pretty ugly and maybe
impractical to work around this hardware issue.  So I'm a little bit
hesitant to add functionality that might have to be removed in the
future.

> Signed-off-by: Vidya Sagar <vidyas@...dia.com>
> Reviewed-by: Jingoo Han <jingoohan1@...il.com>
> ---
> V3:
> * Added 'Reviewed-by: Jingoo Han <jingoohan1@...il.com>'
> 
> V2:
> * Addressed Jingoo's review comment
> * Removed saving 'td' bit information in 'dw_pcie' structure
> 
>  drivers/pci/controller/dwc/pcie-designware.c | 8 ++++++--
>  drivers/pci/controller/dwc/pcie-designware.h | 1 +
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
> index b5e438b70cd5..cbd651b219d2 100644
> --- a/drivers/pci/controller/dwc/pcie-designware.c
> +++ b/drivers/pci/controller/dwc/pcie-designware.c
> @@ -246,6 +246,8 @@ static void dw_pcie_prog_outbound_atu_unroll(struct dw_pcie *pci, u8 func_no,
>  	dw_pcie_writel_ob_unroll(pci, index, PCIE_ATU_UNR_UPPER_TARGET,
>  				 upper_32_bits(pci_addr));
>  	val = type | PCIE_ATU_FUNC_NUM(func_no);
> +	if (pci->version == 0x490A)
> +		val = val | pcie_is_ecrc_enabled() << PCIE_ATU_TD_SHIFT;
>  	val = upper_32_bits(size - 1) ?
>  		val | PCIE_ATU_INCREASE_REGION_SIZE : val;
>  	dw_pcie_writel_ob_unroll(pci, index, PCIE_ATU_UNR_REGION_CTRL1, val);
> @@ -294,8 +296,10 @@ static void __dw_pcie_prog_outbound_atu(struct dw_pcie *pci, u8 func_no,
>  			   lower_32_bits(pci_addr));
>  	dw_pcie_writel_dbi(pci, PCIE_ATU_UPPER_TARGET,
>  			   upper_32_bits(pci_addr));
> -	dw_pcie_writel_dbi(pci, PCIE_ATU_CR1, type |
> -			   PCIE_ATU_FUNC_NUM(func_no));
> +	val = type | PCIE_ATU_FUNC_NUM(func_no);
> +	if (pci->version == 0x490A)
> +		val = val | pcie_is_ecrc_enabled() << PCIE_ATU_TD_SHIFT;
> +	dw_pcie_writel_dbi(pci, PCIE_ATU_CR1, val);
>  	dw_pcie_writel_dbi(pci, PCIE_ATU_CR2, PCIE_ATU_ENABLE);
>  
>  	/*
> diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> index e7f441441db2..b01ef407fd52 100644
> --- a/drivers/pci/controller/dwc/pcie-designware.h
> +++ b/drivers/pci/controller/dwc/pcie-designware.h
> @@ -89,6 +89,7 @@
>  #define PCIE_ATU_TYPE_IO		0x2
>  #define PCIE_ATU_TYPE_CFG0		0x4
>  #define PCIE_ATU_TYPE_CFG1		0x5
> +#define PCIE_ATU_TD_SHIFT		8
>  #define PCIE_ATU_FUNC_NUM(pf)           ((pf) << 20)
>  #define PCIE_ATU_CR2			0x908
>  #define PCIE_ATU_ENABLE			BIT(31)
> -- 
> 2.17.1
>