Subject: Re: [PATCH] arm: cns3xxx: fix writing to wrong PCI registers after alignment

From: Bjorn Helgaas <[email-masked]>
Date: Sat, 29 Dec 2018 19:06:26 -0600

[+cc linux-pci]

On Tue, Dec 18, 2018 at 12:17:43PM +0100, Koen Vandeputte wrote:
> Originally, cns3xxx used it's own functions for mapping, reading and writing registers.
> 
> Commit 802b7c06adc7 ("ARM: cns3xxx: Convert PCI to use generic config accessors")
> removed the internal PCI config write function in favor of the generic one:
> 
> cns3xxx_pci_write_config() --> pci_generic_config_write()
> 
> cns3xxx_pci_write_config() expected aligned addresses, being produced by cns3xxx_pci_map_bus()
> while the generic one pci_generic_config_write() actually expects the real address
> as both the function and hardware are capable of byte-aligned writes.
> 
> This currently leads to pci_generic_config_write() writing
> to the wrong registers on some ocasions.
> 
> First issue seen due to this:
> 
> - driver ath9k gets loaded
> - The driver wants to write value 0xA8 to register PCI_LATENCY_TIMER, located at 0x0D
> - cns3xxx_pci_map_bus() aligns the address to 0x0C
> - pci_generic_config_write() effectively writes 0xA8 into register 0x0C (CACHE_LINE_SIZE)
> 
> This seems to cause some slight instability when certain PCI devices are used.
> 
> Another issue example caused by this this is the PCI bus numbering,
> where the primary bus is higher than the secondary, which is impossible.
> 
> Before:
> 
> 00:00.0 PCI bridge: Cavium, Inc. Device 3400 (rev 01) (prog-if 00 [Normal decode])
>     Flags: bus master, fast devsel, latency 0, IRQ 255
>     Bus: primary=02, secondary=01, subordinate=ff, sec-latency=0
> 
> After fix:
> 
> 00:00.0 PCI bridge: Cavium, Inc. Device 3400 (rev 01) (prog-if 00 [Normal decode])
>     Flags: bus master, fast devsel, latency 0, IRQ 255
>     Bus: primary=00, secondary=01, subordinate=02, sec-latency=0
> 
> And very likely some more ..
> 
> Fix all by omitting the alignment being done in the mapping function.
> 
> Fixes: 802b7c06adc7 ("ARM: cns3xxx: Convert PCI to use generic config accessors")
> Signed-off-by: Koen Vandeputte <[email-masked]>
> CC: Arnd Bergmann <[email-masked]>
> CC: Bjorn Helgaas <[email-masked]>
> CC: Krzysztof Halasa <[email-masked]>
> CC: Olof Johansson <[email-masked]>
> CC: Robin Leblon <[email-masked]>
> CC: Rob Herring <[email-masked]>
> CC: Russell King <[email-masked]>
> CC: Tim Harvey <[email-masked]>
> CC: [email-masked] # v4.0+
> ---
>  arch/arm/mach-cns3xxx/pcie.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mach-cns3xxx/pcie.c b/arch/arm/mach-cns3xxx/pcie.c
> index 318394ed5c7a..5e11ad3164e0 100644
> --- a/arch/arm/mach-cns3xxx/pcie.c
> +++ b/arch/arm/mach-cns3xxx/pcie.c
> @@ -83,7 +83,7 @@ static void __iomem *cns3xxx_pci_map_bus(struct pci_bus *bus,
>  	} else /* remote PCI bus */
>  		base = cnspci->cfg1_regs + ((busno & 0xf) << 20);
>  
> -	return base + (where & 0xffc) + (devfn << 12);
> +	return base + where + (devfn << 12);

802b7c06adc7 replaced cns3xxx_pci_write_config(), which always used
__raw_writel() so it only did 32-bit accesses, with
pci_generic_config_write(), which uses writeb/writew/writel()
depending on the size.

802b7c06adc7 also converted cns3xxx_pci_read_config() from always
using __raw_readl() (a 32-bit access) to using
pci_generic_config_read32(), which also always does a 32-bit access.

This makes me think the cnx3xxx hardware is only capable of 32-bit
accesses, and this patch should change the driver to use
pci_generic_config_write32() instead of pci_generic_config_write() in
addition to the mapping fix above.

What do you think?

I don't think hardware that only supports 32-bit PCI writes can be
quite spec-compliant (see the comments in
pci_generic_config_write32()).  So (1) you may see an additional
dmesg warning when you convert to use it, and (2) it's possible that
there may still be instability related to the corruption caused by
using a 32-bit write when an 8-bit write was intended.

>  }
>  
>  static int cns3xxx_pci_read_config(struct pci_bus *bus, unsigned int devfn,
> -- 
> 2.17.1
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> [email-masked]
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

_______________________________________________
linux-arm-kernel mailing list
[email-masked]
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel