firmware_loader: formalize built-in firmware API

Formalize the built-in firmware with a proper API. This can later
be used by other callers where all they need is built-in firmware.

We export the firmware_request_builtin() call for now only
under the TEST_FIRMWARE symbol namespace as there are no
direct modular users for it. If they pop up they are free
to export it generally. Built-in code always gets access to
the callers and we'll demonstrate a hidden user which has been
lurking in the kernel for a while and the reason why using a
proper API was better long term.

Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20211021155843.1969401-2-mcgrof@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/firmware_loader/builtin/main.c

@@ -0,0 +1,100 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Builtin firmware support */
+
+#include <linux/firmware.h>
+#include "../firmware.h"
+
+/* Only if FW_LOADER=y */
+#ifdef CONFIG_FW_LOADER
+
+extern struct builtin_fw __start_builtin_fw[];
+extern struct builtin_fw __end_builtin_fw[];
+
+static bool fw_copy_to_prealloc_buf(struct firmware *fw,
+				    void *buf, size_t size)
+{
+	if (!buf)
+		return true;
+	if (size < fw->size)
+		return false;
+	memcpy(buf, fw->data, fw->size);
+	return true;
+}
+
+/**
+ * firmware_request_builtin() - load builtin firmware
+ * @fw: pointer to firmware struct
+ * @name: name of firmware file
+ *
+ * Some use cases in the kernel have a requirement so that no memory allocator
+ * is involved as these calls take place early in boot process. An example is
+ * the x86 CPU microcode loader. In these cases all the caller wants is to see
+ * if the firmware was built-in and if so use it right away. This can be used
+ * for such cases.
+ *
+ * This looks for the firmware in the built-in kernel. Only if the kernel was
+ * built-in with the firmware you are looking for will this return successfully.
+ *
+ * Callers of this API do not need to use release_firmware() as the pointer to
+ * the firmware is expected to be provided locally on the stack of the caller.
+ **/
+bool firmware_request_builtin(struct firmware *fw, const char *name)
+{
+	struct builtin_fw *b_fw;
+
+	if (!fw)
+		return false;
+
+	for (b_fw = __start_builtin_fw; b_fw != __end_builtin_fw; b_fw++) {
+		if (strcmp(name, b_fw->name) == 0) {
+			fw->size = b_fw->size;
+			fw->data = b_fw->data;
+			return true;
+		}
+	}
+
+	return false;
+}
+EXPORT_SYMBOL_NS_GPL(firmware_request_builtin, TEST_FIRMWARE);
+
+/**
+ * firmware_request_builtin_buf() - load builtin firmware into optional buffer
+ * @fw: pointer to firmware struct
+ * @name: name of firmware file
+ * @buf: If set this lets you use a pre-allocated buffer so that the built-in
+ *	firmware into is copied into. This field can be NULL. It is used by
+ *	callers such as request_firmware_into_buf() and
+ *	request_partial_firmware_into_buf()
+ * @size: if buf was provided, the max size of the allocated buffer available.
+ *	If the built-in firmware does not fit into the pre-allocated @buf this
+ *	call will fail.
+ *
+ * This looks for the firmware in the built-in kernel. Only if the kernel was
+ * built-in with the firmware you are looking for will this call possibly
+ * succeed. If you passed a @buf the firmware will be copied into it *iff* the
+ * built-in firmware fits into the pre-allocated buffer size specified in
+ * @size.
+ *
+ * This caller is to be used internally by the firmware_loader only.
+ **/
+bool firmware_request_builtin_buf(struct firmware *fw, const char *name,
+				  void *buf, size_t size)
+{
+	if (!firmware_request_builtin(fw, name))
+		return false;
+
+	return fw_copy_to_prealloc_buf(fw, buf, size);
+}
+
+bool firmware_is_builtin(const struct firmware *fw)
+{
+	struct builtin_fw *b_fw;
+
+	for (b_fw = __start_builtin_fw; b_fw != __end_builtin_fw; b_fw++)
+		if (fw->data == b_fw->data)
+			return true;
+
+	return false;
+}
+
+#endif