When "No ST mode" is enabled, endpoint devices can generate TPH headers
but with all steering tags treated as zero. A steering tag of zero is
interpreted as "using the default policy" by the root complex. This is
essential to quantify the benefit of steering tags for some given
workloads.

Co-developed-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Wei Huang <wei.huang2@amd.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
---
 .../admin-guide/kernel-parameters.txt         |  1 +
 drivers/pci/pci-driver.c                      |  7 +++++-
 drivers/pci/pci.c                             | 12 ++++++++++
 drivers/pci/pcie/tph.c                        | 22 +++++++++++++++++++
 include/linux/pci-tph.h                       |  2 ++
 include/linux/pci.h                           |  1 +
 6 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 65581ebd9b50..1b761f062969 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4656,6 +4656,7 @@
 		norid		[S390] ignore the RID field and force use of
 				one PCI domain per PCI function
 		notph		[PCIE] Do not use PCIe TPH
+		nostmode	[PCIE] Force TPH to use No ST Mode
 
 	pcie_aspm=	[PCIE] Forcibly enable or ignore PCIe Active State Power
 			Management.
diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index 9722d070c0ca..abe66541536e 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -324,8 +324,13 @@ static long local_pci_probe(void *_ddi)
 	pci_dev->driver = pci_drv;
 	rc = pci_drv->probe(pci_dev, ddi->id);
 	if (!rc) {
-		if (pci_tph_disabled())
+		if (pci_tph_disabled()) {
 			pcie_tph_disable(pci_dev);
+			return rc;
+		}
+
+		if (pci_tph_nostmode())
+			pcie_tph_set_nostmode(pci_dev);
 
 		return rc;
 	}
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 4cbfd5b53be8..8745ce1c4a9a 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -160,6 +160,9 @@ static bool pcie_ats_disabled;
 /* If set, the PCIe TPH capability will not be used. */
 static bool pcie_tph_disabled;
 
+/* If TPH is enabled, "No ST Mode" will be enforced. */
+static bool pcie_tph_nostmode;
+
 /* If set, the PCI config space of each device is printed during boot. */
 bool pci_early_dump;
 
@@ -175,6 +178,12 @@ bool pci_tph_disabled(void)
 }
 EXPORT_SYMBOL_GPL(pci_tph_disabled);
 
+bool pci_tph_nostmode(void)
+{
+	return pcie_tph_nostmode;
+}
+EXPORT_SYMBOL_GPL(pci_tph_nostmode);
+
 /* Disable bridge_d3 for all PCIe ports */
 static bool pci_bridge_d3_disable;
 /* Force bridge_d3 for all PCIe ports */
@@ -6881,6 +6890,9 @@ static int __init pci_setup(char *str)
 			} else if (!strcmp(str, "notph")) {
 				pr_info("PCIe: TPH is disabled\n");
 				pcie_tph_disabled = true;
+			} else if (!strcmp(str, "nostmode")) {
+				pr_info("PCIe: TPH No ST Mode is enabled\n");
+				pcie_tph_nostmode = true;
 			} else if (!strncmp(str, "cbiosize=", 9)) {
 				pci_cardbus_io_size = memparse(str + 9, &str);
 			} else if (!strncmp(str, "cbmemsize=", 10)) {
diff --git a/drivers/pci/pcie/tph.c b/drivers/pci/pcie/tph.c
index ad58a892792c..fb8e2f920712 100644
--- a/drivers/pci/pcie/tph.c
+++ b/drivers/pci/pcie/tph.c
@@ -13,6 +13,19 @@
 
 #include "../pci.h"
 
+/* Update the ST Mode Select field of TPH Control Register */
+static void set_ctrl_reg_mode_sel(struct pci_dev *pdev, u8 st_mode)
+{
+	u32 reg_val;
+
+	pci_read_config_dword(pdev, pdev->tph_cap + PCI_TPH_CTRL, &reg_val);
+
+	reg_val &= ~PCI_TPH_CTRL_MODE_SEL_MASK;
+	reg_val |= FIELD_PREP(PCI_TPH_CTRL_MODE_SEL_MASK, st_mode);
+
+	pci_write_config_dword(pdev, pdev->tph_cap + PCI_TPH_CTRL, reg_val);
+}
+
 /* Update the TPH Requester Enable field of TPH Control Register */
 static void set_ctrl_reg_req_en(struct pci_dev *pdev, u8 req_type)
 {
@@ -26,6 +39,15 @@ static void set_ctrl_reg_req_en(struct pci_dev *pdev, u8 req_type)
 	pci_write_config_dword(pdev, pdev->tph_cap + PCI_TPH_CTRL, reg_val);
 }
 
+void pcie_tph_set_nostmode(struct pci_dev *pdev)
+{
+	if (!pdev->tph_cap)
+		return;
+
+	set_ctrl_reg_mode_sel(pdev, PCI_TPH_NO_ST_MODE);
+	set_ctrl_reg_req_en(pdev, PCI_TPH_REQ_TPH_ONLY);
+}
+
 void pcie_tph_disable(struct pci_dev *pdev)
 {
 	if (!pdev->tph_cap)
diff --git a/include/linux/pci-tph.h b/include/linux/pci-tph.h
index e0b782bda929..8fce3969277c 100644
--- a/include/linux/pci-tph.h
+++ b/include/linux/pci-tph.h
@@ -11,8 +11,10 @@
 
 #ifdef CONFIG_PCIE_TPH
 void pcie_tph_disable(struct pci_dev *dev);
+void pcie_tph_set_nostmode(struct pci_dev *dev);
 #else
 static inline void pcie_tph_disable(struct pci_dev *dev) {}
+static inline void pcie_tph_set_nostmode(struct pci_dev *dev) {}
 #endif
 
 #endif /* LINUX_PCI_TPH_H */
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 05fbbd9ad6b4..ac58f3919993 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -1841,6 +1841,7 @@ static inline bool pci_aer_available(void) { return false; }
 
 bool pci_ats_disabled(void);
 bool pci_tph_disabled(void);
+bool pci_tph_nostmode(void);
 
 #ifdef CONFIG_PCIE_PTM
 int pci_enable_ptm(struct pci_dev *dev, u8 *granularity);
-- 
2.45.1

On Wed, Jul 17, 2024 at 03:55:05PM -0500, Wei Huang wrote:
> When "No ST mode" is enabled, endpoint devices can generate TPH headers
> but with all steering tags treated as zero. A steering tag of zero is
> interpreted as "using the default policy" by the root complex. This is
> essential to quantify the benefit of steering tags for some given
> workloads.

Capitalize technical terms defined by spec.

> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4656,6 +4656,7 @@
>  		norid		[S390] ignore the RID field and force use of
>  				one PCI domain per PCI function
>  		notph		[PCIE] Do not use PCIe TPH
> +		nostmode	[PCIE] Force TPH to use No ST Mode

Needs a little more context here about what this means.  Users won't
know where to even look for "No ST Mode" unless they have a copy of
the spec.

> +++ b/drivers/pci/pci-driver.c
> @@ -324,8 +324,13 @@ static long local_pci_probe(void *_ddi)
>  	pci_dev->driver = pci_drv;
>  	rc = pci_drv->probe(pci_dev, ddi->id);
>  	if (!rc) {
> -		if (pci_tph_disabled())
> +		if (pci_tph_disabled()) {
>  			pcie_tph_disable(pci_dev);
> +			return rc;
> +		}
> +
> +		if (pci_tph_nostmode())
> +			pcie_tph_set_nostmode(pci_dev);

Same comment here; can we do this outside the probe() path somehow?

>  		return rc;
>  	}
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 4cbfd5b53be8..8745ce1c4a9a 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -160,6 +160,9 @@ static bool pcie_ats_disabled;
>  /* If set, the PCIe TPH capability will not be used. */
>  static bool pcie_tph_disabled;
>  
> +/* If TPH is enabled, "No ST Mode" will be enforced. */
> +static bool pcie_tph_nostmode;
> +
>  /* If set, the PCI config space of each device is printed during boot. */
>  bool pci_early_dump;
>  
> @@ -175,6 +178,12 @@ bool pci_tph_disabled(void)
>  }
>  EXPORT_SYMBOL_GPL(pci_tph_disabled);
>  
> +bool pci_tph_nostmode(void)
> +{
> +	return pcie_tph_nostmode;
> +}
> +EXPORT_SYMBOL_GPL(pci_tph_nostmode);

s/pci/pcie/

Unexport unless it's useful for drivers.

Bjorn

On 7/23/24 17:44, Bjorn Helgaas wrote:
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -4656,6 +4656,7 @@
>>   		norid		[S390] ignore the RID field and force use of
>>   				one PCI domain per PCI function
>>   		notph		[PCIE] Do not use PCIe TPH
>> +		nostmode	[PCIE] Force TPH to use No ST Mode
> 
> Needs a little more context here about what this means.  Users won't
> know where to even look for "No ST Mode" unless they have a copy of
> the spec.
> 

I can certainly add more description to talk about "No ST Mode". Also, 
will "tph_nostmode" be better than "nostmode" in your opinion?

On Thu, Aug 01, 2024 at 11:29:22PM -0500, Wei Huang wrote:
> On 7/23/24 17:44, Bjorn Helgaas wrote:
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -4656,6 +4656,7 @@
> > >   		norid		[S390] ignore the RID field and force use of
> > >   				one PCI domain per PCI function
> > >   		notph		[PCIE] Do not use PCIe TPH
> > > +		nostmode	[PCIE] Force TPH to use No ST Mode
> > 
> > Needs a little more context here about what this means.  Users won't
> > know where to even look for "No ST Mode" unless they have a copy of
> > the spec.
> 
> I can certainly add more description to talk about "No ST Mode". Also, will
> "tph_nostmode" be better than "nostmode" in your opinion?

I don't really care about the parameter name.  I think just spelling
these out once, e.g., "PCIe TLP Processing Hints (TPH)" and "No ST
Mode (Steering Tags must be all zeros)" is probably enough context.