linux-kernel - [PATCH] pci: Make DWORD accesses while saving / restoring LTR state

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Message-Id: <20211222012105.3438916-1-rajatja@google.com>
Date:   Tue, 21 Dec 2021 17:21:05 -0800
From:   Rajat Jain <rajatja@...gle.com>
To:     Bjorn Helgaas <bhelgaas@...gle.com>, kw@...ux.com,
        Rajat Jain <rajatja@...gle.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     rajatxjain@...il.com
Subject: [PATCH] pci: Make DWORD accesses while saving / restoring LTR state

Some devices have an errata such that they only support DWORD accesses
to some registers.

For e.g. this Bayhub O2 device ([VID:DID] = [0x1217:0x8621]) only
supports DWORD accesses to LTR latency registers and L1 PM substates
control registers:
https://github.com/rajatxjain/public_shared/blob/main/OZ711LV2_appnote.pdf

Since L1 PM substate control registers are DWORD sized, and hence their
access in the kernel is already DWORD sized, so we don't need to do
anything for them.

However, the LTR registers being WORD sized, are in need of a solution.
This patch converts the WORD sized accesses to these registers, into
DWORD sized accesses, while saving and restoring them.

Signed-off-by: Rajat Jain <rajatja@...gle.com>
---
 drivers/pci/pci.c       | 24 ++++++++++++++++--------
 drivers/pci/pcie/aspm.c |  1 +
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 3d2fb394986a..efa8cd16827f 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1556,7 +1556,7 @@ static void pci_save_ltr_state(struct pci_dev *dev)
 {
 	int ltr;
 	struct pci_cap_saved_state *save_state;
-	u16 *cap;
+	u32 *cap;
 
 	if (!pci_is_pcie(dev))
 		return;
@@ -1571,25 +1571,33 @@ static void pci_save_ltr_state(struct pci_dev *dev)
 		return;
 	}
 
-	cap = (u16 *)&save_state->cap.data[0];
-	pci_read_config_word(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, cap++);
-	pci_read_config_word(dev, ltr + PCI_LTR_MAX_NOSNOOP_LAT, cap++);
+	/*
+	 * We deliberately do a dword access to save both PCI_LTR_MAX_SNOOP_LAT
+	 * and PCI_LTR_MAX_NOSNOOP_LAT together since some devices only support
+	 * dword accesses to these registers.
+	 */
+	cap = &save_state->cap.data[0];
+	pci_read_config_dword(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, cap);
 }
 
 static void pci_restore_ltr_state(struct pci_dev *dev)
 {
 	struct pci_cap_saved_state *save_state;
 	int ltr;
-	u16 *cap;
+	u32 *cap;
 
 	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_LTR);
 	ltr = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_LTR);
 	if (!save_state || !ltr)
 		return;
 
-	cap = (u16 *)&save_state->cap.data[0];
-	pci_write_config_word(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, *cap++);
-	pci_write_config_word(dev, ltr + PCI_LTR_MAX_NOSNOOP_LAT, *cap++);
+	/*
+	 * We deliberately do a dword access to restore both
+	 * PCI_LTR_MAX_SNOOP_LAT and PCI_LTR_MAX_NOSNOOP_LAT together since
+	 * some devices only support dword accesses to these registers.
+	 */
+	cap = &save_state->cap.data[0];
+	pci_write_config_dword(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, *cap);
 }
 
 /**
diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index 52c74682601a..083f47a7b69b 100644
--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -496,6 +496,7 @@ static void aspm_calc_l1ss_info(struct pcie_link_state *link,
 	encode_l12_threshold(l1_2_threshold, &scale, &value);
 	ctl1 |= t_common_mode << 8 | scale << 29 | value << 16;
 
+	/* Always make DWORD sized accesses to these registers */
 	pci_read_config_dword(parent, parent->l1ss + PCI_L1SS_CTL1, &pctl1);
 	pci_read_config_dword(parent, parent->l1ss + PCI_L1SS_CTL2, &pctl2);
 	pci_read_config_dword(child, child->l1ss + PCI_L1SS_CTL1, &cctl1);
-- 
2.34.1.307.g9b7440fafd-goog