linux-kernel - [PATCH 3.16 162/306] drm/radeon: drop register readback in cayman_cp_int_cntl

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Message-ID: <lsq.1487198500.773098419@decadent.org.uk>
Date:   Wed, 15 Feb 2017 22:41:40 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Alex Deucher" <alexander.deucher@....com>,
        "Lucas Stach" <dev@...xeye.de>,
        "Christian König" <christian.koenig@....com>
Subject: [PATCH 3.16 162/306] drm/radeon: drop register readback in
 cayman_cp_int_cntl_setup

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Lucas Stach <dev@...xeye.de>

commit 537b4b462caa8bfb9726d9695b8e56e2d5e6b41e upstream.

The read is taking a considerable amount of time (about 50us on this
machine). The register does not ever hold anything other than the ring
ID that is updated in this exact function, so there is no need for
the read modify write cycle.

This chops off a big chunk of the time spent in hardirq disabled
context, as this function is called multiple times in the interrupt
handler. With this change applied radeon won't show up in the list
of the worst IRQ latency offenders anymore, where it was a regular
before.

Reviewed-by: Christian König <christian.koenig@....com>
Signed-off-by: Lucas Stach <dev@...xeye.de>
Signed-off-by: Alex Deucher <alexander.deucher@....com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/gpu/drm/radeon/ni.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

--- a/drivers/gpu/drm/radeon/ni.c
+++ b/drivers/gpu/drm/radeon/ni.c
@@ -1333,9 +1333,7 @@ static void cayman_pcie_gart_fini(struct
 void cayman_cp_int_cntl_setup(struct radeon_device *rdev,
 			      int ring, u32 cp_int_cntl)
 {
-	u32 srbm_gfx_cntl = RREG32(SRBM_GFX_CNTL) & ~3;
-
-	WREG32(SRBM_GFX_CNTL, srbm_gfx_cntl | (ring & 3));
+	WREG32(SRBM_GFX_CNTL, RINGID(ring));
 	WREG32(CP_INT_CNTL, cp_int_cntl);
 }