[Intel-gfx] [RFC 7/8] drm/i915: Skip CSB processing on invalid CSB tail

[Jeff McGee]

On Wed, Mar 21, 2018 at 10:26:24AM -0700, jeff.mcgee at intel.com wrote:
> From: Jeff McGee <jeff.mcgee at intel.com>
> 
> Engine reset is fast. A context switch interrupt may be generated just
> prior to the reset such that the top half handler is racing with reset
> post-processing. The handler may set the irq_posted bit again after
> the reset code has cleared it to start fresh. Then the re-enabled
> tasklet will read the CSB head and tail from MMIO, which will be at
> the hardware reset values of 0 and 7 respectively, given that no
> actual CSB event has occurred since the reset. Mayhem then ensues as
> the tasklet starts processing invalid CSB entries.
> 
> We can handle this corner case without adding any new synchronization
> between the irq top half and the reset work item. The tasklet can
> just skip CSB processing if the tail is not sane.
> 
> Signed-off-by: Jeff McGee <jeff.mcgee at intel.com>
> ---
If I drop this patch and substitute https://patchwork.freedesktop.org/patch/211831/
I will see irq_posted get set after reset which causes the first tasklet
run to re-process a previous CSB event and hit GEM_BUG_ON that nothing
was active.
-Jeff