drm/print: Add drm_err_printer()

A simple convienence function that returns a drm_printer which prints
using pr_err()

Changes since v1:
* Make __drm_printfn_err() more consistent with DRM_ERROR() - danvet

Cc: Juston Li <juston.li@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <hwentlan@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190903204645.25487-6-lyude@redhat.com

drivers/gpu/drm/drm_print.c

@@ -147,6 +147,12 @@ void __drm_printfn_debug(struct drm_printer *p, struct va_format *vaf)
 }
 EXPORT_SYMBOL(__drm_printfn_debug);
 
+void __drm_printfn_err(struct drm_printer *p, struct va_format *vaf)
+{
+	pr_err("*ERROR* %s %pV", p->prefix, vaf);
+}
+EXPORT_SYMBOL(__drm_printfn_err);
+
 /**
  * drm_puts - print a const string to a &drm_printer stream
  * @p: the &drm printer

include/drm/drm_print.h

@@ -83,6 +83,7 @@ void __drm_printfn_seq_file(struct drm_printer *p, struct va_format *vaf);
 void __drm_puts_seq_file(struct drm_printer *p, const char *str);
 void __drm_printfn_info(struct drm_printer *p, struct va_format *vaf);
 void __drm_printfn_debug(struct drm_printer *p, struct va_format *vaf);
+void __drm_printfn_err(struct drm_printer *p, struct va_format *vaf);
 
 __printf(2, 3)
 void drm_printf(struct drm_printer *p, const char *f, ...);
@@ -227,6 +228,22 @@ static inline struct drm_printer drm_debug_printer(const char *prefix)
 	return p;
 }
 
+/**
+ * drm_err_printer - construct a &drm_printer that outputs to pr_err()
+ * @prefix: debug output prefix
+ *
+ * RETURNS:
+ * The &drm_printer object
+ */
+static inline struct drm_printer drm_err_printer(const char *prefix)
+{
+	struct drm_printer p = {
+		.printfn = __drm_printfn_err,
+		.prefix = prefix
+	};
+	return p;
+}
+
 /*
  * The following categories are defined:
  *