Revert "drm/amdgpu: Set GTT size equal to TTM mem limit"

We need `amdgpu.gttsize=3072` for the testing on machines with =< 4G RAM.

This reverts commit f1f6f48a.

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -270,6 +270,7 @@ extern int amdgpu_wbrf;
 
 #define AMDGPU_VM_MAX_NUM_CTX			4096
 #define AMDGPU_SG_THRESHOLD			(256*1024*1024)
+#define AMDGPU_DEFAULT_GTT_SIZE_MB		3072ULL /* 3GB by default */
 #define AMDGPU_WAIT_IDLE_TIMEOUT_IN_MS	        3000
 #define AMDGPU_MAX_USEC_TIMEOUT			100000	/* 100 ms */
 #define AMDGPU_FENCE_JIFFIES_TIMEOUT		(HZ / 2)

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

@@ -1956,13 +1956,26 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
 	DRM_INFO("amdgpu: %uM of VRAM memory ready\n",
 		 (unsigned int)(adev->gmc.real_vram_size / (1024 * 1024)));
 
-	/* Compute GTT size, either based on TTM limit
-	 * or whatever the user passed on module init.
-	 */
-	if (amdgpu_gtt_size == -1)
-		gtt_size = ttm_tt_pages_limit() << PAGE_SHIFT;
-	else
+	/* Compute GTT size, either based on 1/2 the size of RAM size
+	 * or whatever the user passed on module init */
+	if (amdgpu_gtt_size == -1) {
+		struct sysinfo si;
+
+		si_meminfo(&si);
+		/* Certain GL unit tests for large textures can cause problems
+		 * with the OOM killer since there is no way to link this memory
+		 * to a process.  This was originally mitigated (but not necessarily
+		 * eliminated) by limiting the GTT size.  The problem is this limit
+		 * is often too low for many modern games so just make the limit 1/2
+		 * of system memory which aligns with TTM. The OOM accounting needs
+		 * to be addressed, but we shouldn't prevent common 3D applications
+		 * from being usable just to potentially mitigate that corner case.
+		 */
+		gtt_size = max((AMDGPU_DEFAULT_GTT_SIZE_MB << 20),
+			       (u64)si.totalram * si.mem_unit / 2);
+	} else {
 		gtt_size = (uint64_t)amdgpu_gtt_size << 20;
+	}
 
 	/* Initialize GTT memory pool */
 	r = amdgpu_gtt_mgr_init(adev, gtt_size);