[Intel-gfx] [PATCH 3/4] drm/i915: opt-out CPU and WC mmaps from FBC

Chris Wilson chris at chris-wilson.co.uk
Thu Mar 24 19:31:54 UTC 2016

On Thu, Mar 24, 2016 at 04:16:11PM -0300, Paulo Zanoni wrote:
> FBC and the frontbuffer tracking infrastructure were designed assuming
> that user space applications would follow a specific set of rules
> regarding frontbuffer management and mmapping. I recently discovered
> that current user space is not exactly following these rules: my
> investigation led me to the conclusion that the generic backend from
> SNA - used by SKL and the other new platforms without a specific
> backend - is not issuing sw_finish/dirty_fb IOCTLs when using the CPU
> and WC mmaps. I discovered this when running lightdm: I would type the
> password and nothing would appear on the screen unless I moved the
> mouse over the place where the letters were supposed to appear.

Yes, that is a kernel bug. The protocol we said the kernel would follow
is to disable FBC/WC when userspace marks the object for writing by the
CPU and would only reestablish FBC/WC upon dirtyfb.

Chris Wilson, Intel Open Source Technology Centre

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