drm/i915: Cache the error string

Currently, we convert the error state into a string every time we read
from sysfs (and sysfs reads in page size (4KiB) chunks). We do try to
window the string and only capture the portion that is being read, but
that means that we must always convert up to the window to find the
start. For a very large error state bordering on EXEC_OBJECT_CAPTURE
abuse, this is noticeable as it degrades to O(N^2)!

As we do not have a convenient hook for sysfs open(), and we would like
to keep the lazy conversion into a string, do the conversion of the
whole string on the first read and keep the string until the error state
is freed.

v2: Don't double advance simple_read_from_buffer
v3: Due to extreme pain of lack of vrealloc, use a scatterlist
v4: Keep the forward iterator loosely cached
v5: Stylistic improvements to reduce patch size

Reported-by: Jason Ekstrand <jason@jlekstrand.net>
Testcase: igt/gem_exec_capture/many*
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123132325.26541-1-chris@chris-wilson.co.uk

Author: ickle

drivers/gpu/drm/i915/i915_debugfs.c

@@ -943,30 +943,30 @@ static int i915_gem_fence_regs_info(struct seq_file *m, void *data)
 static ssize_t gpu_state_read(struct file *file, char __user *ubuf,
 			      size_t count, loff_t *pos)
 {
-	struct i915_gpu_state *error = file->private_data;
-	struct drm_i915_error_state_buf str;
+	struct i915_gpu_state *error;
 	ssize_t ret;
-	loff_t tmp;
+	void *buf;
 
+	error = file->private_data;
 	if (!error)
 		return 0;
 
-	ret = i915_error_state_buf_init(&str, error->i915, count, *pos);
-	if (ret)
-		return ret;
+	/* Bounce buffer required because of kernfs __user API convenience. */
+	buf = kmalloc(count, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
 
-	ret = i915_error_state_to_str(&str, error);
-	if (ret)
+	ret = i915_gpu_state_copy_to_buffer(error, buf, *pos, count);
+	if (ret <= 0)
 		goto out;
 
-	tmp = 0;
-	ret = simple_read_from_buffer(ubuf, count, &tmp, str.buf, str.bytes);
-	if (ret < 0)
-		goto out;
+	if (!copy_to_user(ubuf, buf, ret))
+		*pos += ret;
+	else
+		ret = -EFAULT;
 
-	*pos = str.start + ret;
 out:
-	i915_error_state_buf_release(&str);
+	kfree(buf);
 	return ret;
 }