Re: Device probing and driver attach

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Fri, Nov 03, 2017 at 07:46:04PM +0100, Rocky Hotas wrote:
> - In principle, if one built a custom kernel including *only* the drivers
> needed by its current machine, would the boot time get significantly
> reduced?

Well, assuming any modernish bus with decent device enumeration
functionality, the answer is pretty much "it doesn't matter". Most of
the probe functions match things like bus/vendor IDs against a table.
There are very few PCI devices that are more involved like checking the
PCI config space (i.e. rtk vs re) and require many additional cycles. So
even with a thousand probe functions run with one microsecond teach, we
are talking about < 1 milli second per device. Given that the actual
hardware configuration often contains delay loops, probing simply doesn't
matter.

> - When a BIOS does not perform this operation, is during the
> autoconfiguration that device BARs are written by the OS?

There are two different parts here:
(1) Probing the size of a PCI BAR. This is done by the OS for general
accounting.
(2) Reserving and/or assigning space for devices. This is generally
assumptions on x86 and many other platforms to be handled by the
firmware, with some fallback code i.e. for hot plugging bridges.

Joerg