linux-kernel - Re: [PATCH v2] [RFC PATCH] PCI: Update LTR threshold based on LTRME bit

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Date:   Tue, 5 Apr 2022 11:08:00 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Prasad Malisetty <quic_pmaliset@...cinc.com>
Cc:     agross@...nel.org, bjorn.andersson@...aro.org,
        lorenzo.pieralisi@....com, robh@...nel.org, kw@...ux.com,
        bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rajatja@...gle.com, refactormyself@...il.com,
        quic_vbadigan@...cinc.com, quic_ramkri@...cinc.com,
        manivannan.sadhasivam@...aro.org, swboyd@...omium.org
Subject: Re: [PATCH v2] [RFC PATCH] PCI: Update LTR threshold based on LTRME
 bit

On Tue, Mar 08, 2022 at 12:29:09AM +0530, Prasad Malisetty wrote:
> Update LTR threshold scale and value based on LTRME (Latency
> Tolenrance Reporting Mechanism) from device capabilities.

s/Tolenrance/Tolerance/

I'm not familiar with "LTRME" and it doesn't appear in the PCI spec.
Please use the PCIe terminology whenever possible.

> In ASPM driver, LTR threshold scale and value is updating
> based on tcommon_mode and t_poweron values. In kioxia NVMe,
> L1.2 is failing due to LTR threshold scale and value is
> greater values than max snoop/non snoop value.
> 
> In general, updated LTR threshold scale and value should be
> less than max snoop/non snoop value to enter the device
> into L1.2 state.

This affects all PCIe devices, so please include the PCIe spec
citation that leads to this change.

Please wrap the commit log to fill 75 columns.

Thanks a lot for looking at this!  L1.2 and LTR is real problem area,
and it would be great if we could get it all sorted out.

> Signed-off-by: Prasad Malisetty <quic_pmaliset@...cinc.com>
> 
> ---
> Changes since v1:
> 	- Added missing variable declaration in v1 patch.
> ---
>  drivers/pci/pcie/aspm.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index a96b742..a67746c 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -463,6 +463,7 @@ static void aspm_calc_l1ss_info(struct pcie_link_state *link,
>  	u32 val1, val2, scale1, scale2;
>  	u32 t_common_mode, t_power_on, l1_2_threshold, scale, value;
>  	u32 ctl1 = 0, ctl2 = 0;
> +	u32 cap;
>  	u32 pctl1, pctl2, cctl1, cctl2;
>  	u32 pl1_2_enables, cl1_2_enables;
>  
> @@ -499,9 +500,14 @@ static void aspm_calc_l1ss_info(struct pcie_link_state *link,
>  	 * Table 5-11.  T(POWER_OFF) is at most 2us and T(L1.2) is at
>  	 * least 4us.
>  	 */
> -	l1_2_threshold = 2 + 4 + t_common_mode + t_power_on;
> -	encode_l12_threshold(l1_2_threshold, &scale, &value);
> -	ctl1 |= t_common_mode << 8 | scale << 29 | value << 16;
> +	pcie_capability_read_dword(child, PCI_EXP_DEVCAP2, &cap);
> +	if (!(cap & PCI_EXP_DEVCAP2_LTR)) {
> +		l1_2_threshold = 2 + 4 + t_common_mode + t_power_on;
> +		encode_l12_threshold(l1_2_threshold, &scale, &value);
> +		ctl1 |= scale << 29 | value << 16;
> +	}
> +
> +	ctl1 |= t_common_mode;
>  
>  	/* Some broken devices only support dword access to L1 SS */
>  	pci_read_config_dword(parent, parent->l1ss + PCI_L1SS_CTL1, &pctl1);
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member 
> of Code Aurora Forum, hosted by The Linux Foundation
>