Hi Jean,
Am 18.02.2016 um 09:59 schrieb Jean Delvare:
Hi Daniel,
On Tue, 16 Feb 2016 22:14:12 +0100, Daniel Vetter wrote:
On Tue, Feb 16, 2016 at 09:33:51PM +0100, Christian König wrote:
At least for Radeon and Amdgpu the current situation is actually what we
want.
However I still find it confusing
that full userptr support is under #if defined(CONFIG_MMU_NOTIFIER),
and not under #if defined(CONFIG_RADEON_USERPTR), resp. #if
defined(CONFIG_AMDGPU_USERPTR). It means that full userptr support may
be included even if these options are disabled.
Which is perfectly fine. Userptr support should be enabled when MMU_NOTIFIER
is available.
How this becomes available is a different story. You can explicitly enable
it which then pulls in the MMU_NOTIFIER dependency or you just enable it
when you have the notfier anyway because of some other dependency.
That we have two options doing the same is just a matter of branching of
amdgpu from radeon and not cleaning up the configuration options. So feel
free to cleaning this up and write a patch which makes pulling in
MMU_NOTIFIER as a general DRM option.
Yeah I like that flow. Jean, if you want to bring i915 into alignment with
radone by adding a I915_USERPTR option that selects MMU_NOTIFIER (probably
default y since vulkan needs this), then I very much want will merge it.
Maybe I was not clear enough in my original post, but I am really
advocating for FEWER kconfig options, not more. Plus I just explained
why I think the radeon and amdgpu drivers do it wrong, so I'm certainly
not going to align i915 on that.
Distro kernels pretty much all select MMU_NOTIFIER already for unrelated
reasons, but it's good to be less suprising for everyone who builds their
own custom kernel.
The current situation is actually very surprising and this is what I
would like to change. The options of one driver depending on choices
made somewhere else is utterly confusing. CONFIG_RADEON_USERPTR and
CONFIG_AMDGPU_USERPTR give the user a little more control where i915
gives none, but that's only in one direction (you can force full
userptr support in, but you can't force it out.)
As I said, that is perfectly fine and exactly what we want.
I'm still waiting for someone to explain why we can't just
unconditionally select MMU_NOTIFIER in these 3 drivers and be done with
it. That's less work at kernel configuration time AND fewer
preprocessing conditionals within the code, so easier/better testing
coverage and more readable code. We would need a very good reason for
NOT doing it.
Adding the MMU Notifier causes overhead in the MM which people want to
avoid when they don't need userptr support.
We used to select MMU_NOTIFIER unconditionally before and explicitly
added this option on user request. So yes it is clearly necessary.
The additional userptr support code in the drivers are negligible so
always enabling the code when the MMU Notifier is available is also
perfectly fine.
So if you want to clean it up make a common DRM option which selects MMU
Notifier if it isn't already selected.
Regards,
Christian.
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