console: Implement an environment var to control the pager

The 'pager' environment variable can be used to set the number of lines
on the display. Provide a callback for this.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/pager.c

@@ -7,6 +7,7 @@
 
 #define LOG_CATEGORY LOGC_CONSOLE
 
+#include <env.h>
 #include <errno.h>
 #include <malloc.h>
 #include <pager.h>
@@ -161,6 +162,33 @@ void pager_reset(struct pager *pag)
 	pag->line_count = 0;
 }
 
+static int on_pager(const char *name, const char *value, enum env_op op,
+		    int flags)
+{
+	struct pager *pag = gd_pager();
+	int new_page_len;
+
+	if (!IS_ENABLED(CONFIG_CONSOLE_PAGER) || !pag)
+		return 0;
+
+	switch (op) {
+	case env_op_create:
+	case env_op_overwrite:
+		if (value) {
+			new_page_len = simple_strtoul(value, NULL, 16);
+			pager_set_page_len(pag, new_page_len);
+		}
+		break;
+	case env_op_delete:
+		/* Reset to default when deleted */
+		pager_set_page_len(pag, CONFIG_CONSOLE_PAGER_LINES);
+		break;
+	}
+
+	return 0;
+}
+U_BOOT_ENV_CALLBACK(pager, on_pager);
+
 void pager_uninit(struct pager *pag)
 {
 	abuf_uninit(&pag->buf);

doc/usage/environment.rst

@@ -335,6 +335,9 @@ netretry
     Useful on scripts which control the retry operation
     themselves.
 
+pager
+    Decimal number of visible lines on the display, or serial console.
+
 rng_seed_size
     Size of random value added to device-tree node /chosen/rng-seed.
     This variable is given as a decimal number.

include/env_callback.h

@@ -75,6 +75,12 @@
 #define DFU_CALLBACK
 #endif
 
+#ifdef CONFIG_CONSOLE_PAGER
+#define PAGER_CALLBACK	"pager:pager,"
+#else
+#define PAGER_CALLBACK
+#endif
+
 /*
  * This list of callback bindings is static, but may be overridden by defining
  * a new association in the ".callbacks" environment variable.
@@ -88,6 +94,7 @@
 	DFU_CALLBACK \
 	"loadaddr:loadaddr," \
 	SILENT_CALLBACK \
+	PAGER_CALLBACK \
 	"stdin:console,stdout:console,stderr:console," \
 	"serial#:serialno," \
 	CFG_ENV_CALLBACK_LIST_STATIC