Re: [PATCH 1/3] PCI: Introduce PCI software bridge common logic

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

Hello Russell,

On Wed, 1 Aug 2018 10:21:19 +0100, Russell King - ARM Linux wrote:


>    All PCI devices must treat Configuration Space write operations
>    to reserved registers as no-ops; that is, the access must be
>    completed normally on the bus and the data discarded. Read
>    accesses to reserved or unimplemented registers must be completed
>    normally and a data value of 0 returned.
> 
> (eg) PCI status:
>    Reserved bits should be read-only and return zero when read.
>    A one bit is reset (if it is not read-only) whenever the register
>    is written, and the write data in the corresponding bit location
>    is a 1.
> 
> [which is why doing the read-modify-write action that some host
>  bridges that only support 32-bit accesses is dangerous - it leads
>  to various status bits being inadvertently reset.]

Speaking of this, the generic pci_generic_config_write32() function
indeed does this incorrectly, and prints a warning.

However, I just looked at the pci-thunder-pem code, and it seems to
correctly account for those W1C bits, by having an explicit list of
registers and their W1C bits (see thunder_pem_bridge_w1c_bits). This
isn't specific to the Thunder PCIe controller at all, and would benefit
from being made generic, no ?

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com