util: Add a way to set the min_buffer_size in linear_alloc

The default value remains 2048, which is also used for rounding up
any user provided size.

The option is useful in cases where there's a better idea of the
amount of data that's going to be used.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25279>

src/util/tests/linear_test.cpp

@@ -70,3 +70,22 @@ TEST(LinearAlloc, AvoidWasteAfterLargeAlloc)
 
    ralloc_free(ctx);
 }
+
+TEST(LinearAlloc, Options)
+{
+   void *ctx = ralloc_context(NULL);
+
+   linear_opts opts = {};
+   opts.min_buffer_size = 8192;
+
+   linear_ctx *lin_ctx = linear_context_with_opts(ctx, &opts);
+
+   /* Assert allocations spanning the first 8192 bytes are contiguous. */
+   char *first = (char *)linear_alloc_child(lin_ctx, 1024);
+   for (int i = 1; i < 8; i++) {
+      char *ptr = (char *)linear_alloc_child(lin_ctx, 1024);
+      EXPECT_EQ(ptr - first, 1024 * i);
+   }
+
+   ralloc_free(ctx);
+}