drm/radeon/kms: read back register before writing in IIO.

This fixes RH bugzilla #527874.

On resume the atom posting wasn't working, however vbe posting was
going fine, after 2 weeks over irc, and 8 hrs with the hardware,
I tracked it down to the memory device table and it access the MC
registers via IIO, it appears the rv515 atom iio table might not
be fully functional, so adding a readback before doing a write
either provides enough delay to make things resume correctly.

Thanks to Peng Huang at Red Hat for coming to Brisbane.

Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/gpu/drm/radeon/atom.c b/drivers/gpu/drm/radeon/atom.c
index 901befe03da2..d67c42555ab9 100644
--- a/drivers/gpu/drm/radeon/atom.c
+++ b/drivers/gpu/drm/radeon/atom.c
@@ -107,6 +107,7 @@ static uint32_t atom_iio_execute(struct atom_context *ctx, int base,
                         base += 3;
                         break;
                 case ATOM_IIO_WRITE:
+                        (void)ctx->card->reg_read(ctx->card, CU16(base + 1));
                         ctx->card->reg_write(ctx->card, CU16(base + 1), temp);
                         base += 3;
                         break;