linux-kernel - [PATCH] drm/msm/adreno: De-spaghettify the use of memory barriers

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Date: Wed, 08 May 2024 19:46:31 +0200
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>, 
 Abhinav Kumar <quic_abhinavk@...cinc.com>, 
 Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, 
 Marijn Suijten <marijn.suijten@...ainline.org>, 
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>
Cc: Rob Clark <robdclark@...omium.org>, linux-arm-msm@...r.kernel.org, 
 dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org, 
 linux-kernel@...r.kernel.org, Konrad Dybcio <konrad.dybcio@...aro.org>
Subject: [PATCH] drm/msm/adreno: De-spaghettify the use of memory barriers

Memory barriers help ensure instruction ordering, NOT time and order
of actual write arrival at other observers (e.g. memory-mapped IP).
On architectures employing weak memory ordering, the latter can be a
giant pain point, and it has been as part of this driver.

Moreover, the gpu_/gmu_ accessors already use non-relaxed versions of
readl/writel, which include r/w (respectively) barriers.

Replace the barriers with a readback that ensures the previous writes
have exited the write buffer (as the CPU must flush the write to the
register it's trying to read back) and subsequently remove the hack
introduced in commit b77532803d11 ("drm/msm/a6xx: Poll for GBIF unhalt
status in hw_init").

Fixes: b77532803d11 ("drm/msm/a6xx: Poll for GBIF unhalt status in hw_init")
Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
---
 drivers/gpu/drm/msm/adreno/a6xx_gmu.c |  5 ++---
 drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 14 ++++----------
 2 files changed, 6 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
index 0e3dfd4c2bc8..4135a53b55a7 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
@@ -466,9 +466,8 @@ static int a6xx_rpmh_start(struct a6xx_gmu *gmu)
 	int ret;
 	u32 val;
 
-	gmu_write(gmu, REG_A6XX_GMU_RSCC_CONTROL_REQ, 1 << 1);
-	/* Wait for the register to finish posting */
-	wmb();
+	gmu_write(gmu, REG_A6XX_GMU_RSCC_CONTROL_REQ, BIT(1));
+	gmu_read(gmu, REG_A6XX_GMU_RSCC_CONTROL_REQ);
 
 	ret = gmu_poll_timeout(gmu, REG_A6XX_GMU_RSCC_CONTROL_ACK, val,
 		val & (1 << 1), 100, 10000);
diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
index 973872ad0474..0acbc38b8e70 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
@@ -1713,22 +1713,16 @@ static int hw_init(struct msm_gpu *gpu)
 	}
 
 	/* Clear GBIF halt in case GX domain was not collapsed */
+	gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);
+	gpu_read(gpu, REG_A6XX_GBIF_HALT);
 	if (adreno_is_a619_holi(adreno_gpu)) {
-		gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);
 		gpu_write(gpu, REG_A6XX_RBBM_GPR0_CNTL, 0);
-		/* Let's make extra sure that the GPU can access the memory.. */
-		mb();
+		gpu_read(gpu, REG_A6XX_RBBM_GPR0_CNTL);
 	} else if (a6xx_has_gbif(adreno_gpu)) {
-		gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);
 		gpu_write(gpu, REG_A6XX_RBBM_GBIF_HALT, 0);
-		/* Let's make extra sure that the GPU can access the memory.. */
-		mb();
+		gpu_read(gpu, REG_A6XX_RBBM_GBIF_HALT);
 	}
 
-	/* Some GPUs are stubborn and take their sweet time to unhalt GBIF! */
-	if (adreno_is_a7xx(adreno_gpu) && a6xx_has_gbif(adreno_gpu))
-		spin_until(!gpu_read(gpu, REG_A6XX_GBIF_HALT_ACK));
-
 	gpu_write(gpu, REG_A6XX_RBBM_SECVID_TSB_CNTL, 0);
 
 	if (adreno_is_a619_holi(adreno_gpu))

---
base-commit: 93a39e4766083050ca0ecd6a3548093a3b9eb60c
change-id: 20240508-topic-adreno-a2d199cd4152

Best regards,
-- 
Konrad Dybcio <konrad.dybcio@...aro.org>