Re: [PATCH V13 3/6] PCI: loongson: Don't access unexisting devices

[Bjorn Helgaas <helgaas@kernel.org>]

On Sat, Apr 30, 2022 at 04:48:43PM +0800, Huacai Chen wrote:
> On LS2K/LS7A, some unexisting devices don't return 0xffffffff when
> scanning. This is a hardware flaw but we can only avoid it by software
> now.

s/unexisting/non-existant/ (many occurrences: subject line, commit
log, comments below)

What happens in other situations that normally cause Unsupported
Request or similar errors?  For example, memory reads/writes to a
device in D3hot should cause an Unsupported Request error.  I'm
wondering whether other error handling assumptions might be broken
on LS2K/LS7A.

> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
>  drivers/pci/controller/pci-loongson.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
> index adbfa4a2330f..48316daa1f23 100644
> --- a/drivers/pci/controller/pci-loongson.c
> +++ b/drivers/pci/controller/pci-loongson.c
> @@ -138,6 +138,8 @@ static void __iomem *pci_loongson_map_bus(struct pci_bus *bus, unsigned int devf
>  			       int where)
>  {
>  	unsigned char busnum = bus->number;
> +	unsigned int device = PCI_SLOT(devfn);
> +	unsigned int function = PCI_FUNC(devfn);
>  	struct loongson_pci *priv = pci_bus_to_loongson_pci(bus);
>  
>  	if (pci_is_root_bus(bus))
> @@ -147,8 +149,13 @@ static void __iomem *pci_loongson_map_bus(struct pci_bus *bus, unsigned int devf
>  	 * Do not read more than one device on the bus other than
>  	 * the host bus. For our hardware the root bus is always bus 0.
>  	 */
> -	if (priv->data->flags & FLAG_DEV_FIX &&
> -			!pci_is_root_bus(bus) && PCI_SLOT(devfn) > 0)
> +	if ((priv->data->flags & FLAG_DEV_FIX) && bus->self) {
> +		if (!pci_is_root_bus(bus) && (device > 0))
> +			return NULL;
> +	}
> +
> +	/* Don't access unexisting devices */
> +	if (pci_is_root_bus(bus) && (device >= 9 && device <= 20 && function > 0))

Yuck.  This is pretty nasty magic.  If this is something that might be
fixed in future versions of the hardware, maybe you should factor this
out into a function pointer in loongson_pci_data or something.

>  		return NULL;
>  
>  	/* CFG0 can only access standard space */
> -- 
> 2.27.0
>