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MIPS: Cobalt: Explain GT64111 early PCI fixup

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Properly document why changing PCI Class Code for GT64111 device to Host
Bridge is required as important details were after 20 years forgotten.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
This commit is contained in:
Pali Rohár 2021-11-02 18:12:59 +01:00 committed by Thomas Bogendoerfer
parent f69fa4c81b
commit 36de23a4c5
1 changed files with 15 additions and 0 deletions
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15
arch/mips/pci/fixup-cobalt.c
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@ -36,6 +36,21 @@
#define VIA_COBALT_BRD_ID_REG 0x94
#define VIA_COBALT_BRD_REG_to_ID(reg) ((unsigned char)(reg) >> 4)
/*
* Default value of PCI Class Code on GT64111 is PCI_CLASS_MEMORY_OTHER (0x0580)
* instead of PCI_CLASS_BRIDGE_HOST (0x0600). Galileo explained this choice in
* document "GT-64111 System Controller for RC4640, RM523X and VR4300 CPUs",
* section "6.5.3 PCI Autoconfiguration at RESET":
*
* Some PCs refuse to configure host bridges if they are found plugged into
* a PCI slot (ask the BIOS vendors why...). The "Memory Controller" Class
* Code does not cause a problem for these non-compliant BIOSes, so we used
* this as the default in the GT-64111.
*
* So fix the incorrect default value of PCI Class Code. More details are on:
* https://lore.kernel.org/r/20211102154831.xtrlgrmrizl5eidl@pali/
* https://lore.kernel.org/r/20211102150201.GA11675@alpha.franken.de/
*/
static void qube_raq_galileo_early_fixup(struct pci_dev *dev)
{
if (dev->devfn == PCI_DEVFN(0, 0) &&