Re: [PATCH v13 3/6] PCI: Refactor capability search into common macros

From: Hans Zhang
Date: Wed Jul 16 2025 - 12:05:53 EST



On 2025/7/16 06:47, Bjorn Helgaas wrote:
On Sun, Jun 08, 2025 at 12:14:02AM +0800, Hans Zhang wrote:
The PCI Capability search functionality is duplicated across the PCI core
and several controller drivers. The core's current implementation requires
fully initialized PCI device and bus structures, which prevents controller
drivers from using it during early initialization phases before these
structures are available.

Move the Capability search logic into a header-based macro that accepts a
config space accessor function as an argument. This enables controller
drivers to perform Capability discovery using their early access
mechanisms prior to full device initialization while sharing the
Capability search code.

Convert the existing PCI core Capability search implementation to use this
new macro. Controller drivers can later use the same macros with their
early access mechanisms while maintaining the existing protection against
infinite loops through preserved TTL checks.

The ttl parameter was originally an additional safeguard to prevent
infinite loops in corrupted config space. However, the
PCI_FIND_NEXT_CAP_TTL() macro already enforces a TTL limit internally.
Removing redundant ttl handling simplifies the interface while maintaining
the safety guarantee. This aligns with the macro's design intent of
encapsulating TTL management.

This is a big gulp, but I think I like it :) It really enables some
nice cleanups later.

-static u8 __pci_find_next_cap_ttl(struct pci_bus *bus, unsigned int devfn,
- u8 pos, int cap, int *ttl)
-{
- u8 id;
- u16 ent;
-
- pci_bus_read_config_byte(bus, devfn, pos, &pos);
-
- while ((*ttl)--) {
- if (pos < PCI_STD_HEADER_SIZEOF)
- break;
- pos = ALIGN_DOWN(pos, 4);
- pci_bus_read_config_word(bus, devfn, pos, &ent);
-
- id = FIELD_GET(PCI_CAP_ID_MASK, ent);
- if (id == 0xff)
- break;
- if (id == cap)
- return pos;
- pos = FIELD_GET(PCI_CAP_LIST_NEXT_MASK, ent);
- }
- return 0;
-}
-
static u8 __pci_find_next_cap(struct pci_bus *bus, unsigned int devfn,
u8 pos, int cap)
{
- int ttl = PCI_FIND_CAP_TTL;
-
- return __pci_find_next_cap_ttl(bus, devfn, pos, cap, &ttl);
+ return PCI_FIND_NEXT_CAP_TTL(pci_bus_read_config, pos, cap, bus, devfn);
}
u8 pci_find_next_capability(struct pci_dev *dev, u8 pos, int cap)
@@ -554,42 +527,11 @@ EXPORT_SYMBOL(pci_bus_find_capability);
*/
u16 pci_find_next_ext_capability(struct pci_dev *dev, u16 start, int cap)
{
- u32 header;
- int ttl;
- u16 pos = PCI_CFG_SPACE_SIZE;
-
- /* minimum 8 bytes per capability */
- ttl = (PCI_CFG_SPACE_EXP_SIZE - PCI_CFG_SPACE_SIZE) / 8;
-
if (dev->cfg_size <= PCI_CFG_SPACE_SIZE)
return 0;
- if (start)
- pos = start;
-
- if (pci_read_config_dword(dev, pos, &header) != PCIBIOS_SUCCESSFUL)
- return 0;
-
- /*
- * If we have no capabilities, this is indicated by cap ID,
- * cap version and next pointer all being 0.
- */
- if (header == 0)
- return 0;
-
- while (ttl-- > 0) {
- if (PCI_EXT_CAP_ID(header) == cap && pos != start)
- return pos;
-
- pos = PCI_EXT_CAP_NEXT(header);
- if (pos < PCI_CFG_SPACE_SIZE)
- break;
-
- if (pci_read_config_dword(dev, pos, &header) != PCIBIOS_SUCCESSFUL)
- break;
- }
-
- return 0;
+ return PCI_FIND_NEXT_EXT_CAPABILITY(pci_bus_read_config, start, cap,
+ dev->bus, dev->devfn);
}
EXPORT_SYMBOL_GPL(pci_find_next_ext_capability);
@@ -649,7 +591,7 @@ EXPORT_SYMBOL_GPL(pci_get_dsn);
static u8 __pci_find_next_ht_cap(struct pci_dev *dev, u8 pos, int ht_cap)
{
- int rc, ttl = PCI_FIND_CAP_TTL;
+ int rc;
u8 cap, mask;
if (ht_cap == HT_CAPTYPE_SLAVE || ht_cap == HT_CAPTYPE_HOST)
@@ -657,8 +599,8 @@ static u8 __pci_find_next_ht_cap(struct pci_dev *dev, u8 pos, int ht_cap)
else
mask = HT_5BIT_CAP_MASK;
- pos = __pci_find_next_cap_ttl(dev->bus, dev->devfn, pos,
- PCI_CAP_ID_HT, &ttl);
+ pos = PCI_FIND_NEXT_CAP_TTL(pci_bus_read_config, pos,
+ PCI_CAP_ID_HT, dev->bus, dev->devfn);
while (pos) {
rc = pci_read_config_byte(dev, pos + 3, &cap);
if (rc != PCIBIOS_SUCCESSFUL)
@@ -667,9 +609,10 @@ static u8 __pci_find_next_ht_cap(struct pci_dev *dev, u8 pos, int ht_cap)
if ((cap & mask) == ht_cap)
return pos;
- pos = __pci_find_next_cap_ttl(dev->bus, dev->devfn,
- pos + PCI_CAP_LIST_NEXT,
- PCI_CAP_ID_HT, &ttl);
+ pos = PCI_FIND_NEXT_CAP_TTL(pci_bus_read_config,
+ pos + PCI_CAP_LIST_NEXT,
+ PCI_CAP_ID_HT, dev->bus,
+ dev->devfn);
}
return 0;
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index e7d31ed56731..46fb6b5a854e 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -2,6 +2,8 @@
#ifndef DRIVERS_PCI_H
#define DRIVERS_PCI_H
+#include <linux/align.h>
+#include <linux/bitfield.h>
#include <linux/pci.h>
struct pcie_tlp_log;
@@ -93,6 +95,89 @@ bool pcie_cap_has_rtctl(const struct pci_dev *dev);
int pci_bus_read_config(void *priv, unsigned int devfn, int where, u32 size,
u32 *val);
+/* Standard Capability finder */
+/**
+ * PCI_FIND_NEXT_CAP_TTL - Find a PCI standard capability

I don't think "_TTL" is relevant in the macro name here. I see it
copied the previous __pci_find_next_cap_ttl() name; "ttl" *was*
relevant there, but it isn't anymore.


Dear Bjorn,

Thank you very much for your reply.

The _TTL suffix will be removed.


I would like this a lot better if it could be implemented as a
function, but I assume it has to be a macro for some varargs reason.


Yes. The macro definitions used in combination with the previous review opinions of Ilpo.

+ * @read_cfg: Function pointer for reading PCI config space
+ * @start: Starting position to begin search
+ * @cap: Capability ID to find
+ * @args: Arguments to pass to read_cfg function
+ *
+ * Iterates through the capability list in PCI config space to find
+ * @cap. Implements TTL (time-to-live) protection against infinite loops.
+ *
+ * Returns: Position of the capability if found, 0 otherwise.
+ */
+#define PCI_FIND_NEXT_CAP_TTL(read_cfg, start, cap, args...) \
+({ \
+ int __ttl = PCI_FIND_CAP_TTL; \
+ u8 __id, __found_pos = 0; \
+ u8 __pos = (start); \
+ u16 __ent; \
+ \
+ read_cfg(args, __pos, 1, (u32 *)&__pos); \
+ \
+ while (__ttl--) { \
+ if (__pos < PCI_STD_HEADER_SIZEOF) \
+ break; \

I guess this is just lifted directly from __pci_find_next_cap_ttl(),
but wow, I wish it weren't quite *so* subtle. Totally fine though.

Maybe this could be split into one patch for standard capabilities and
a second for extended capabilities, just so the connection between
this and __pci_find_next_cap_ttl() would be clearer.


I will split the two patches.

+ \
+ __pos = ALIGN_DOWN(__pos, 4); \
+ read_cfg(args, __pos, 2, (u32 *)&__ent); \
+ \
+ __id = FIELD_GET(PCI_CAP_ID_MASK, __ent); \
+ if (__id == 0xff) \
+ break; \
+ \
+ if (__id == (cap)) { \
+ __found_pos = __pos; \
+ break; \
+ } \
+ \
+ __pos = FIELD_GET(PCI_CAP_LIST_NEXT_MASK, __ent); \
+ } \
+ __found_pos; \
+})
+
+/* Extended Capability finder */
+/**
+ * PCI_FIND_NEXT_EXT_CAPABILITY - Find a PCI extended capability

Can this be named similarly to the above? PCI_FIND_NEXT_CAP_TTL() and
PCI_FIND_NEXT_EXT_CAPABILITY() seem needlessly different.

No problem at all.


PCI_FIND_NEXT_CAP() and PCI_FIND_NEXT_EXT_CAP()?

Will change.


+ * @read_cfg: Function pointer for reading PCI config space
+ * @start: Starting position to begin search (0 for initial search)
+ * @cap: Extended capability ID to find
+ * @args: Arguments to pass to read_cfg function
+ *
+ * Searches the extended capability space in PCI config registers
+ * for @cap. Implements TTL protection against infinite loops using
+ * a calculated maximum search count.
+ *
+ * Returns: Position of the capability if found, 0 otherwise.
+ */
+#define PCI_FIND_NEXT_EXT_CAPABILITY(read_cfg, start, cap, args...) \

It would be *really* nice if you could make this fit nicely in 80
columns as PCI_FIND_NEXT_CAP_TTL() does.


Will change.

Best regards,
Hans

+({ \
+ u16 __pos = (start) ?: PCI_CFG_SPACE_SIZE; \
+ u16 __found_pos = 0; \
+ int __ttl, __ret; \
+ u32 __header; \
+ \
+ __ttl = (PCI_CFG_SPACE_EXP_SIZE - PCI_CFG_SPACE_SIZE) / 8; \
+ while (__ttl-- > 0 && __pos >= PCI_CFG_SPACE_SIZE) { \
+ __ret = read_cfg(args, __pos, 4, &__header); \
+ if (__ret != PCIBIOS_SUCCESSFUL) \
+ break; \
+ \
+ if (__header == 0) \
+ break; \
+ \
+ if (PCI_EXT_CAP_ID(__header) == (cap) && __pos != start) { \
+ __found_pos = __pos; \
+ break; \
+ } \
+ \
+ __pos = PCI_EXT_CAP_NEXT(__header); \
+ } \
+ __found_pos; \
+})
+
/* Functions internal to the PCI core code */
#ifdef CONFIG_DMI
--
2.25.1