pci_cfg_space_size_ext(9)

💅 pci_cfg_space_size_ext(9) 💅

okay bestie, so this one is straight out of the Kernel API Manual (yes, that manual, the big scary one — you're gonna be fine, i promise 🫶), and it's all about figuring out how much config space a PCI device is actually packing. lowkey iconic detective work tbh 🔍✨

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🌸 NAME 🌸

pci_cfg_space_size_ext — get the configuration space size of the PCI device. that's it, that's the whole vibe check. 💖

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✨ SYNOPSIS ✨

int pci_cfg_space_size_ext (struct pci_dev *dev );

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🎀 ARGUMENTS 🎀

• dev — the PCI device. yeah. that's the whole argument. she's low maintenance, we love her for it. 👑

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🦋 DESCRIPTION 🦋

okay so here's the tea. regular PCI devices have 256 bytes of config space — cute, compact, no notes. but PCI-X 2 and PCI Express devices have 4096 bytes — main character energy, way more room to work with. 💅🔥

and like… here's the part where it gets messy. even if the device is capable of the big 4096-byte situation, that doesn't mean we can access it. 😭 and why, you ask? two reasons on the table:

• maybe we don't have a way to generate extended config space accesses — like, the plumbing just isn't there, bestie.
• or the device is behind a reverse Express bridge — yeah. that's a thing. and it's giving obstacle. 🙈

so what do we do?? we do a vibe check, obviously: we try reading the dword at 0x100, and that value must either be 0 or a valid extended capability header. if it plays nice, great. if not, well — that's the whole test. 🧚⭐

no cap: this is a probe, not a promise. the read is how we find out if extended config space is actually reachable, so don't skip straight to assuming the big space is yours. 🫡

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👑 SEE ALSO 👑

oh you want the whole friend group? say less. 💜 these all live in the kernel file ./drivers/pci/probe.c:

• __pci_size_bars(9)
• __pci_read_base(9)
• pcie_get_link_speed(9)
• pci_ea_fixed_busnrs(9)
• pci_ext_cfg_is_aliased(9)
• pci_intx_mask_broken(9)
• pci_setup_device(9)
• pcie_relaxed_ordering_enabled(9)
• pci_release_dev(9)
• pci_scan_slot(9)
• pci_scan_child_bus_extend(9)
• pci_scan_child_bus(9)
• pcibios_root_bridge_prepare(9)
• pci_rescan_bus_bridge_resize(9)
• pci_rescan_bus(9)

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and that's the whole page, bestie — go read some config space and slay. 🎀✨

sign-off szn: this little number comes from the ./drivers/pci directory, dated October 2026. 🗓️💖 periodt.