diff --git a/graphics/mapper/4.0/IMapper.hal b/graphics/mapper/4.0/IMapper.hal
index f5df04b..85c7c81 100644
--- a/graphics/mapper/4.0/IMapper.hal
+++ b/graphics/mapper/4.0/IMapper.hal
@@ -202,6 +202,9 @@
      * the buffer. If the bytesPerStride is unknown or variable, a value of -1
      * should be returned.
      *
+     * The locked buffer must adhere to the format requested at allocation time
+     * in the BufferDescriptorInfo.
+     *
      * @param buffer Buffer to lock.
      * @param cpuUsage CPU usage flags to request. See +ndk
      *     libnativewindow#AHardwareBuffer_UsageFlags for possible values.