Re: [dpdk-dev] [dpdk-techboard] RFC: DPDK drivers for DPDK bus types

[Thomas Monjalon <thomas@monjalon.net>]

11/03/2021 16:19, Bruce Richardson:
> Hi all,
> 
> looking for input here into the area of bus-type drivers and interaction
> with other drivers in DPDK.
> 
> By way of context, I'm looking at extending the vdev support in the
> "raw/ioat" driver (file raw/ioat/idxd_vdev.c) to make it more user
> friendly.  These devices are accessed by DPDK via nodes in /dev and
> paths in /sys, with the vdev parameters being passed identifying the
> particular devices to use. However, the presence of these devices can be
> detected at runtime by a scan of /dev and /sys, and so it's easy enough
> to implement a custom bus-type driver in DPDK to detect these, rather
> than having the user pass in vdev parameters (which can get awkward to
> use as the number of devices increases).

I agree. vdev bus should be used for creating device from the void.
If the device has its roots in the system (HW or SW), there should be
a bus for that.


> However, looking through a few other drivers in the "bus" directory, it
> appears that scanning system paths, i.e. /sys, is fairly common, so I'm
> wondering if it's possible to have some sharing of functionality here.
> Unfortunately, the use of /sys in each of these drivers I've looked at
> seem sufficiently different to me that I've not immediately seen a
> common level of abstraction we can use. Therefore I'm looking for
> suggestions here that those in the community might have.

Not sure it's worth looking for such sharing between bus.


> On a related note, I'm also concerned about the need for a single device
> type, e.g. one used by DPDK and shared with the kernel, to require two
> separate drivers to work together to support it - a bus driver for
> scanning and a type-specific driver for the actual functional
> implementation. Can we not find a way to reduce the number of drivers
> needing to be supported?

The bus driver is managing the device life.
The device driver implements a functional class.
I don't see what to save.
Maybe you are biased because the rawdev class is fake.


> Following on from this, and if we can't find a good way of doing a
> generic driver for scanning /sys nodes, I wonder if there is value in
> providing a "generic" bus implementation in DPDK, as a catch-all for
> device drivers which need their own custom probing, but that do not
> neatly fall into the other types. The way this might work is to have the
> scanning and probing of devices left entirely to the device driver
> implementation itself. For example, rather than creating an idxd
> bus driver, it would be easier and more self-contained to have the
> rawdev driver itself able to perform scanning and probing - keeping the
> code all in one place. All the bus driver would have to do is maintain a
> list of drivers and found devices reported by the individual driver
> after they have done their own probing.

So you mean there is a single user of the bus,
so the implementation could be moved into the device driver,
relying on a fake bus?


> Other possible candidates to
> think about that might be able to use their own probing from a generic
> bus might be, e.g. af_xdp driver, or a TAP or memif driver.

These devices don't exist naturally in the system,
so I think they should be vdev.