Am 04.04.2017 um 12:43 schrieb Daniel Stone:
Hi,
On 4 April 2017 at 11:27, Christopher James Halse Rogers
<chris@xxxxxxxxxxxxxx> wrote:
On 4 April 2017 6:31:12 pm AEST, Daniel Vetter <daniel@xxxxxxxx> wrote:
This seems like an awful specific special case. Why can't you do the
entire dance upfront, i.e. import buffer, addfb2? None of that has any
visible effect, and it will also allow you to check runtime constraints
which can't be covered with this here.
No, because addfb2 doesn't (or, rather, didn't) actually check any runtime constraints.
The problem only appeared when the buffer is actually *used* in a modeset - otherwise I could do a (reasonably) cheap import/addfb/render on exporter/read out on importer dance to detect it.
To be clear - this is trying to solve the problem “how can I tell if it's safe to addfb/pageflip rather than do a GL copy to a GPU-local buffer and flip from that?”.
If I could guarantee that I'd only ever run on 4.13-or-later kernels (I think that's when the previous patches will land?), then this would indeed be mostly unnecessary. It would save me a bunch of addfb calls that would predictably fail, but they're cheap.
Yeah, the ABI is that AddFB2 should fail hard on something which can
never be used as a framebuffer. The fact it didn't is a bug rather
than a behavioural change per se ...
I agree on that, but the problem Christopher tries to solve looks a bit
different from my perspective.
He wants to know if a driver can scan out from a foreign BO or if he
needs to copy the content.
The problem I see with this patch is that the kernel doesn't look like
the right place for this decision to make.
Regards,
Christian.
Cheers,
Daniel
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