From: Saeed Mahameed saeed@kernel.org Date: Mon, 04 Oct 2021 15:30:21 -0700
On Mon, 2021-10-04 at 16:59 +0200, Andrew Lunn wrote:
On Fri, Oct 01, 2021 at 05:17:10PM +0200, Alexander Lobakin wrote:
From: Jian Shen shenjian15@huawei.com Date: Wed, 29 Sep 2021 23:50:47 +0800
Hi,
For the prototype of netdev_features_t is u64, and the number of netdevice feature bits is 64 now. So there is no space to introduce new feature bit.
This patchset try to solve it by change the prototype of netdev_features_t from u64 to bitmap. With this change, it's necessary to introduce a set of bitmap operation helpers for netdev features. Meanwhile, the functions which use netdev_features_t as return value are also need to be changed, return the result as an output parameter.
With above changes, it will affect hundreds of files, and all the nic drivers. To make it easy to be reviewed, split the changes to 167 patches to 5 parts.
If you leave the current feature field set (features, hw_features etc.) as is and just add new ones as bitmaps -- I mean, to place only newly added features there -- you won't have to change this in hundreds of drivers.
That makes things messy for the future. Two different ways to express the same thing. And it is a trap waiting for developers to fall into. Is this a new feature or an old feature bit? Should i add it to the old or new bitmap? Will the compiler error out if i get it wrong, or silently accept it?
Another option is to introduce new fields as bitmaps and mirror all features there, but also keep the current ones. This implies some code duplication -- to keep both sets in sync -- but it will also allow to avoid such diffstats. Developers could switch their drivers one-by-one then, and once they finish converting,
Which will never happen. Most developers will say, why bother, it works as it is, i'm too lazy. And many drivers don't have an active developer, and so won't get converted.
Yes it is a big patchset, but at the end, we get a uniform API which is future proof, and no traps waiting for developers to fall into.
I agree, i had to visit this topic a year ago or so, and the only conclusion was is to solve this the hard way, introduce a totally new mechanism, the safest way is to remove old netdev_features_t fields from netdev and add new ones (both names and types), so compiler will catch you if you missed to convert a place.
Makes sense! I'm more a fan of "total conversions" rather than keeping both old/new, just wasn't sure about how long it would take to collect acks. On the other hand, there is probably no need to wait for every single driver team: no real logic changes, sole convertion stuff to bitmaps.
Anyway, we ran out of bits for both netdev_features_t and priv_flags, so such changes need to be done.
maybe hide the implementation details and abstract it away from drivers using getters and manipulation APIs, it is not that bad since drivers are already not supposed to modify netdev_features directly.
That can also be actual, the only thing is that we need to wrap only basic access to netdev_features_t type itself as we have a set of fields (hw, vlan, gso etc.) to work with.
Andrew
Thanks, Al