New subject: [Intel-gfx] [PATCH] agp/intel: Reinforce the barrier after GTT updates

10 Apr 2020

From: Chris Wilson ickle@x201s.alporthouse.com
After changing the timing between GTT updates and execution on the GPU,
we started seeing sporadic failures on Ironlake. These were narrowed
down to being an insufficiently strong enough barrier/delay after
updating the GTT and scheduling execution on the GPU. By forcing the
uncached read, and adding the missing barrier for the singular
insert_page (relocation paths), the sporadic failures go away.
Fixes: 983d308cb8f6 ("agp/intel: Serialise after GTT updates")
Signed-off-by: Chris Wilson chris@chris-wilson.co.uk
Cc: stable@vger.kernel.org # v4.0+
---
 drivers/char/agp/intel-gtt.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/char/agp/intel-gtt.c b/drivers/char/agp/intel-gtt.c
index 66a62d17a3f5..3d42fc4290bc 100644
--- a/drivers/char/agp/intel-gtt.c
+++ b/drivers/char/agp/intel-gtt.c
@@ -846,6 +846,7 @@ void intel_gtt_insert_page(dma_addr_t addr,
    		   unsigned int flags)
 {
    intel_private.driver->write_entry(addr, pg, flags);
+	readl(intel_private.gtt + pg);
    if (intel_private.driver->chipset_flush)
    	intel_private.driver->chipset_flush();
 }
@@ -871,7 +872,7 @@ void intel_gtt_insert_sg_entries(struct sg_table *st,
    		j++;
    	}
    }
-	wmb();
+	readl(intel_private.gtt + j - 1);
    if (intel_private.driver->chipset_flush)
    	intel_private.driver->chipset_flush();
 }
@@ -1105,6 +1106,7 @@ static void i9xx_cleanup(void)
static void i9xx_chipset_flush(void)
 {
+	wmb();
    if (intel_private.i9xx_flush_page)
    	writel(1, intel_private.i9xx_flush_page);
 }
-- 
2.20.1


    