drm: Add a helper function for printing a debugfs_regset32.

The debugfs_regset32 is nice to use for reducing boilerplate in
dumping a bunch of regs in debugfs, but we also want to be able to
print to dmesg them at runtime for driver debugging.  drm_printer lets
us format debugfs and the printk the same way.

v2: Add some kerneldoc for the function (requested by danvet)
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190220210343.28157-1-eric@anholt.net
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com> (v1)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_print.c

@@ -253,3 +253,31 @@ void drm_err(const char *format, ...)
 	va_end(args);
 }
 EXPORT_SYMBOL(drm_err);
+
+/**
+ * drm_print_regset32 - print the contents of registers to a
+ * &drm_printer stream.
+ *
+ * @p: the &drm printer
+ * @regset: the list of registers to print.
+ *
+ * Often in driver debug, it's useful to be able to either capture the
+ * contents of registers in the steady state using debugfs or at
+ * specific points during operation.  This lets the driver have a
+ * single list of registers for both.
+ */
+void drm_print_regset32(struct drm_printer *p, struct debugfs_regset32 *regset)
+{
+	int namelen = 0;
+	int i;
+
+	for (i = 0; i < regset->nregs; i++)
+		namelen = max(namelen, (int)strlen(regset->regs[i].name));
+
+	for (i = 0; i < regset->nregs; i++) {
+		drm_printf(p, "%*s = 0x%08x\n",
+			   namelen, regset->regs[i].name,
+			   readl(regset->base + regset->regs[i].offset));
+	}
+}
+EXPORT_SYMBOL(drm_print_regset32);

include/drm/drm_print.h

@@ -30,6 +30,7 @@
 #include <linux/printk.h>
 #include <linux/seq_file.h>
 #include <linux/device.h>
+#include <linux/debugfs.h>
 
 /**
  * DOC: print
@@ -84,6 +85,7 @@ void __drm_printfn_debug(struct drm_printer *p, struct va_format *vaf);
 __printf(2, 3)
 void drm_printf(struct drm_printer *p, const char *f, ...);
 void drm_puts(struct drm_printer *p, const char *str);
+void drm_print_regset32(struct drm_printer *p, struct debugfs_regset32 *regset);
 
 __printf(2, 0)
 /**