linux-kernel - Re: [PATCH v3] PCI/ASPM: Update LTR threshold based upon reported max latencies

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Message-ID: <47d89a85-5f10-ebf9-1d62-fb886319b2c4@quicinc.com>
Date:   Thu, 2 Jun 2022 15:29:46 +0530
From:   Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
CC:     <helgaas@...nel.org>, <linux-pci@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_vbadigan@...cinc.com>, <quic_ramkri@...cinc.com>,
        <swboyd@...omium.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
        "Saheed O. Bolarinwa" <refactormyself@...il.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rajat Jain <rajatja@...gle.com>, <vidyas@...dia.com>,
        <kenny@...ix.com>
Subject: Re: [PATCH v3] PCI/ASPM: Update LTR threshold based upon reported max
 latencies


On 6/2/2022 1:59 PM, Manivannan Sadhasivam wrote:
> On Wed, Jun 01, 2022 at 05:57:53PM +0530, Krishna Chaitanya Chundru wrote:
>> [+cc kenny, vidya]
>>
>> On 6/1/2022 5:53 PM, Krishna chaitanya chundru wrote:
>>> In ASPM driver, LTR threshold scale and value is updating based on
> s/is/are
>
> s/updating/updated
>
>>> 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
> s/is/are
>
>>> value.
>>>
>>> Based on PCIe r4.1, sec 5.5.1, L1.2 substate must be entered when
>>> reported snoop/no-snoop values is greather than or equal to
>>> LTR_L1.2_THRESHOLD value.
>>>
>>> Suggested-by: Prasad Malisetty  <quic_pmaliset@...cinc.com>
>>> Signed-off-by: Krishna chaitanya chundru <quic_krichai@...cinc.com>
> If you are inheriting the patch from Prasad, then you should still give the
> authorship to him (unless the patch has changed significantly). You can add
> your S-o-b tag to convey that you are carrying the patch from him.
Thanks mani for pointing this, I will modify this in next patch.
>>> ---
>>>
>>> I am takking this patch forward as prasad is no more working with our org.
>>>
>>> Changes since v2:
>>> 	- Replaced LTRME logic with max snoop/no-snoop latencies check.
>>> Changes since v1:
>>> 	- Added missing variable declaration in v1 patch
>>> ---
>>>    drivers/pci/pcie/aspm.c | 22 +++++++++++++++++++++-
>>>    1 file changed, 21 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
>>> index a96b742..4a15e50 100644
>>> --- a/drivers/pci/pcie/aspm.c
>>> +++ b/drivers/pci/pcie/aspm.c
>>> @@ -465,10 +465,19 @@ static void aspm_calc_l1ss_info(struct pcie_link_state *link,
>>>    	u32 ctl1 = 0, ctl2 = 0;
>>>    	u32 pctl1, pctl2, cctl1, cctl2;
>>>    	u32 pl1_2_enables, cl1_2_enables;
>>> +	int ltr;
> This could be u16 too.
Will change in the next patch
>>> +	u16 max_snoop_lat = 0, max_nosnoop_lat = 0;
> No need to initialize these variables.
I will update these in next patch.
>>>    	if (!(link->aspm_support & ASPM_STATE_L1_2_MASK))
>>>    		return;
>>> +	ltr = pci_find_ext_capability(child, PCI_EXT_CAP_ID_LTR);
>>> +	if (!ltr)
>>> +		return;
> Is this capability implemented always?

Based up on spec 4.1, sec 5.5 Ports that support the L1.2 substate for 
ASPM L1 must support this.

And there is already a check in this function  if there is no L1.2 
support the function is returning.

>>> +
>>> +	pci_read_config_word(child, ltr + PCI_LTR_MAX_SNOOP_LAT, &max_snoop_lat);
>>> +	pci_read_config_word(child, ltr + PCI_LTR_MAX_NOSNOOP_LAT, &max_nosnoop_lat);
>>> +
>>>    	/* Choose the greater of the two Port Common_Mode_Restore_Times */
>>>    	val1 = (parent_l1ss_cap & PCI_L1SS_CAP_CM_RESTORE_TIME) >> 8;
>>>    	val2 = (child_l1ss_cap & PCI_L1SS_CAP_CM_RESTORE_TIME) >> 8;
>>> @@ -501,7 +510,18 @@ static void aspm_calc_l1ss_info(struct pcie_link_state *link,
>>>    	 */
>>>    	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;
>>> +
>>> +	/*
>>> +	 * If the max snoop and no snoop latencies are '0', then avoid updating scale
>>> +	 * and value.
>>> +	 *
> This looks fine but...
>
>>> +	 * Based on PCIe r4.1, sec 5.5.1, L1.2 substate must be entered when reported
>>> +	 * snoop/no-snoop values is greather than or equal to LTR_L1.2_THRESHOLD value.
> s/is/are
>
> What about this? What if the snoop/nosnoop latencies are not equal to zero and
> lower than LTR_L1.2_THRESHOLD?
>
> Thanks,
> Mani

Will address this in next patch.

Thanks,

Krishna Chaitanya.

>>> +	 */
>>> +	if ((max_snoop_lat == 0) && (max_nosnoop_lat == 0))
>>> +		ctl1 |= t_common_mode << 8;
>>> +	else
>>> +		ctl1 |= t_common_mode << 8 | scale << 29 | value << 16;
>>>    	/* Some broken devices only support dword access to L1 SS */
>>>    	pci_read_config_dword(parent, parent->l1ss + PCI_L1SS_CTL1, &pctl1);