The end of user-space mode setting? [LWN.net]

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By Jonathan Corbet
January 13, 2010

It has now been a year since kernel mode setting (KMS) went into the mainline. KMS moves control of low-level graphics processor modes into the kernel and away from user-space drivers, with a number of associated advantages. Initially only the Intel driver supported KMS, but it has found its way into the Radeon and Nouveau drivers. Now developers are beginning to talk about eliminating user-space mode setting support entirely.

On the Nouveau front, Ben Skeggs posted a patch to remove non-KMS support, saying:

The non-KMS paths are messy, and lets face it, rotting badly. IMO the KMS code is stable enough now that we can continue without the UMS crutch, and indeed, the KMS code supports a lot more chipsets (particularly on GF8 and up) than the UMS code ever will.

The main objection to the removal of this code is that BSD-based systems do not support KMS, but the current driver does not work on those systems anyway. So, while this patch has not found its way to the mainline, it would not be surprising if that happened before the 2.6.34 release.

At about the same time, some Intel driver developers started to ask whether non-KMS support could be dropped. There, too, it seems that the user-space mode setting code is unloved and proving hard to maintain. This code looks like it will remain an unwelcome guest for a while, though; Linus is in no hurry to remove it, and Dave Airlie is even more reluctant:

I'm in the 2-3 years at a minimum, with at least one kernel with no serious regressions in Intel KMS, which we haven't gotten close to yet. I'm not even sure the Intel guys are taking stable seriously enough yet. So far I don't think there is one kernel release (even stable) that works on all Intel chipsets without backporting patches.

So the removal of non-KMS support from the Intel driver is being held up by concerns about the stability of the KMS code. But there is a bigger issue as well: Intel support has been in the kernel for years, so there are plenty of systems which are dependent on user-space mode setting. That means that the support needs to be maintained for long enough to be sure of not breaking those systems. Nouveau, instead, has the advantage of not having been in the mainline until now, so the same regression concerns do not apply. There are advantages, sometimes, to being the latecomer.

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