drm: drm_printer: Add printer for devcoredump

Add a drm printer suitable for use with the read callback for
devcoredump or other suitable buffer based output format that
isn't otherwise covered by seq_file.

v2: Add improved documentation per Daniel Vetter

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/drm_print.c

@@ -30,6 +30,80 @@
 #include <drm/drmP.h>
 #include <drm/drm_print.h>
 
+void __drm_printfn_coredump(struct drm_printer *p, struct va_format *vaf)
+{
+	struct drm_print_iterator *iterator = p->arg;
+	ssize_t len;
+
+	if (!iterator->remain)
+		return;
+
+	/* Figure out how big the string will be */
+	len = snprintf(NULL, 0, "%pV", vaf);
+
+	if (iterator->offset < iterator->start) {
+		char *buf;
+		ssize_t copy;
+
+		if (iterator->offset + len <= iterator->start) {
+			iterator->offset += len;
+			return;
+		}
+
+		/* Print the string into a temporary buffer */
+		buf = kmalloc(len + 1,
+			GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY);
+		if (!buf)
+			return;
+
+		snprintf(buf, len + 1, "%pV", vaf);
+
+		copy = len - (iterator->start - iterator->offset);
+
+		if (copy > iterator->remain)
+			copy = iterator->remain;
+
+		/* Copy out the bit of the string that we need */
+		memcpy(iterator->data,
+			buf + (iterator->start - iterator->offset), copy);
+
+		iterator->offset = iterator->start + copy;
+		iterator->remain -= copy;
+
+		kfree(buf);
+	} else {
+		char *buf;
+		ssize_t pos = iterator->offset - iterator->start;
+
+		if (len < iterator->remain) {
+			snprintf(((char *) iterator->data) + pos,
+				iterator->remain, "%pV", vaf);
+
+			iterator->offset += len;
+			iterator->remain -= len;
+
+			return;
+		}
+
+		/* Print the string into a temporary buffer */
+		buf = kmalloc(len + 1,
+			GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY);
+		if (!buf)
+			return;
+
+		snprintf(buf, len + 1, "%pV", vaf);
+
+		/* Copy out the remaining bits */
+		memcpy(iterator->data + pos, buf, iterator->remain);
+
+		iterator->offset += iterator->remain;
+		iterator->remain = 0;
+
+		kfree(buf);
+	}
+}
+EXPORT_SYMBOL(__drm_printfn_coredump);
+
 void __drm_printfn_seq_file(struct drm_printer *p, struct va_format *vaf)
 {
 	seq_printf(p->arg, "%pV", vaf);

include/drm/drm_print.h

@@ -73,6 +73,7 @@ struct drm_printer {
 	const char *prefix;
 };
 
+void __drm_printfn_coredump(struct drm_printer *p, struct va_format *vaf);
 void __drm_printfn_seq_file(struct drm_printer *p, struct va_format *vaf);
 void __drm_printfn_info(struct drm_printer *p, struct va_format *vaf);
 void __drm_printfn_debug(struct drm_printer *p, struct va_format *vaf);
@@ -104,6 +105,70 @@ drm_vprintf(struct drm_printer *p, const char *fmt, va_list *va)
 #define drm_printf_indent(printer, indent, fmt, ...) \
 	drm_printf((printer), "%.*s" fmt, (indent), "\t\t\t\t\tX", ##__VA_ARGS__)
 
+/**
+ * struct drm_print_iterator - local struct used with drm_printer_coredump
+ * @data: Pointer to the devcoredump output buffer
+ * @start: The offset within the buffer to start writing
+ * @remain: The number of bytes to write for this iteration
+ */
+struct drm_print_iterator {
+	void *data;
+	ssize_t start;
+	ssize_t remain;
+	/* private: */
+	ssize_t offset;
+};
+
+/**
+ * drm_coredump_printer - construct a &drm_printer that can output to a buffer
+ * from the read function for devcoredump
+ * @iter: A pointer to a struct drm_print_iterator for the read instance
+ *
+ * This wrapper extends drm_printf() to work with a dev_coredumpm() callback
+ * function. The passed in drm_print_iterator struct contains the buffer
+ * pointer, size and offset as passed in from devcoredump.
+ *
+ * For example::
+ *
+ *	void coredump_read(char *buffer, loff_t offset, size_t count,
+ *		void *data, size_t datalen)
+ *	{
+ *		struct drm_print_iterator iter;
+ *		struct drm_printer p;
+ *
+ *		iter.data = buffer;
+ *		iter.start = offset;
+ *		iter.remain = count;
+ *
+ *		p = drm_coredump_printer(&iter);
+ *
+ *		drm_printf(p, "foo=%d\n", foo);
+ *	}
+ *
+ *	void makecoredump(...)
+ *	{
+ *		...
+ *		dev_coredumpm(dev, THIS_MODULE, data, 0, GFP_KERNEL,
+ *			coredump_read, ...)
+ *	}
+ *
+ * RETURNS:
+ * The &drm_printer object
+ */
+static inline struct drm_printer
+drm_coredump_printer(struct drm_print_iterator *iter)
+{
+	struct drm_printer p = {
+		.printfn = __drm_printfn_coredump,
+		.arg = iter,
+	};
+
+	/* Set the internal offset of the iterator to zero */
+	iter->offset = 0;
+
+	return p;
+}
+
 /**
  * drm_seq_file_printer - construct a &drm_printer that outputs to &seq_file
  * @f:  the &struct seq_file to output to