Re: [PATCH v4 2/3] PCI: Allow specifying devices using a base bus and path of devfns

[Randy Dunlap]

Hi,

On 06/22/2018 12:43 PM, Logan Gunthorpe wrote:
> When specifying PCI devices on the kernel command line using a
> BDF, the bus numbers can change when adding or replacing a device,
> changing motherboard firmware, or applying kernel parameters like
> pci=assign-buses. When this happens, it is usually undesirable to
> apply whatever command line tweak to the wrong device.
> 
> Therefore, it is useful to be able to specify devices with a base
> bus number and the path of devfns needed to get to it. (Similar to
> the "device scope" structure in the Intel VT-d spec, Section 8.3.1.)
> 
> Thus, we add an option to specify devices in the following format:
> 
> [<domain>:]<bus>:<slot>.<func>[/<slot>.<func>]*

Please explain the trailing '*'.  I looked thru the code and it doesn't
seem to look for it or care.

> 
> The path can be any segment within the PCI hierarchy of any length and
> determined through the use of 'lspci -t'. When specified this way, it is
> less likely that a renumbered bus will result in a valid device specification
> and the tweak won't be applied to the wrong device.
> 
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Reviewed-by: Stephen Bates <sbates@xxxxxxxxxxxx>
> Acked-by: Christian KÃnig <christian.koenig@xxxxxxx>
> Reviewed-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |   8 +-
>  drivers/pci/pci.c                               | 117 ++++++++++++++++++++----
>  2 files changed, 103 insertions(+), 22 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index e783bcefadac..a69947d9e14e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3000,7 +3000,7 @@
>  				or a set of devices (<pci_dev>). These are
>  				specified in one of the following formats:
>  
> -				[<domain>:]<bus>:<slot>.<func>
> +				[<domain>:]<bus>:<slot>.<func>[/<slot>.<func>]*
>  				pci:<vendor>:<device>[:<subvendor>:<subdevice>]
>  
>  				Note: the first format specifies a PCI
> @@ -3009,7 +3009,11 @@
>  				firmware changes, or due to changes caused
>  				by other kernel parameters. If the
>  				domain is left unspecified, it is
> -				taken to be zero. The second format
> +				taken to be zero. Optionally, a path
> +				to a device through multiple slot/function
> +				addresses can be specified after the base
> +				address (this is more robust against
> +				renumbering issues). The second format
>  				selects devices using IDs from the
>  				configuration space which may match multiple
>  				devices in the system.


thanks,
-- 
~Randy