drm: aux ->transfer() can return 0, deal with it

commit a1f5524a66ff6284d1380cdd7723de82698ff9d3 upstream.

Restore the correct behaviour (as in check msg.reply) when aux
->transfer() returns 0. It got removed in
commit 82922da39190 ("drm/dp_helper: Retry aux transactions on all errors")

Now I can actually dump the "entire" DPCD on a Dell UP2314Q with
ddrescue. It has some offsets in the DPCD that can't be read
for some resaon, all you get is defers. Previously ddrescue would
just give up at the first unredable offset on account of
read() returning 0 means EOF. Here's the ddrescue log
for the interested:
0x00000000  0x00001400  +
0x00001400  0x00000030  -
0x00001430  0x000001D0  +
0x00001600  0x00000030  -
0x00001630  0x0001F9D0  +
0x00021000  0x00000001  -
0x00021001  0x000DEFFF  +

Cc: Lyude <cpaul@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch
Fixes: 82922da39190 ("drm/dp_helper: Retry aux transactions on all errors")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index eeaf5a7c3aa767a6676b9b6054a020b3fea87445..67b28f8018d279426cf0866317e0b3216a127f26 100644 (file)
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -203,7 +203,7 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 
                ret = aux->transfer(aux, &msg);
 
-               if (ret > 0) {
+               if (ret >= 0) {
                        native_reply = msg.reply & DP_AUX_NATIVE_REPLY_MASK;
                        if (native_reply == DP_AUX_NATIVE_REPLY_ACK) {
                                if (ret == size)