drm/dp: Allow signals to interrupt drm_aux-dev reads/writes

Let's be nice and interrupt the dpcd aux-dev reads/writes when there's
a signal pending. Much nicer if the user can hit ^C instead of having to
sit around waiting for the read/write to finish.

time dd if=/dev/drm_dp_aux0 bs=$((1024*1024))
^C

before:
 real	0m34.681s
 user	0m0.003s
 sys	0m6.880s

after:
 real	0m0.222s
 user	0m0.006s
 sys	0m0.057s

Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461786225-7790-1-git-send-email-ville.syrjala@linux.intel.com

drivers/gpu/drm/drm_dp_aux_dev.c

@@ -159,6 +159,12 @@ static ssize_t auxdev_read(struct file *file, char __user *buf, size_t count,
 		uint8_t localbuf[DP_AUX_MAX_PAYLOAD_BYTES];
 		ssize_t todo = min_t(size_t, bytes_pending, sizeof(localbuf));
 
+		if (signal_pending(current)) {
+			res = num_bytes_processed ?
+				num_bytes_processed : -ERESTARTSYS;
+			goto out;
+		}
+
 		res = drm_dp_dpcd_read(aux_dev->aux, *offset, localbuf, todo);
 		if (res <= 0) {
 			res = num_bytes_processed ? num_bytes_processed : res;
@@ -202,6 +208,12 @@ static ssize_t auxdev_write(struct file *file, const char __user *buf,
 		uint8_t localbuf[DP_AUX_MAX_PAYLOAD_BYTES];
 		ssize_t todo = min_t(size_t, bytes_pending, sizeof(localbuf));
 
+		if (signal_pending(current)) {
+			res = num_bytes_processed ?
+				num_bytes_processed : -ERESTARTSYS;
+			goto out;
+		}
+
 		if (__copy_from_user(localbuf,
 				     buf + num_bytes_processed, todo)) {
 			res = num_bytes_processed ?