drm/ast: Don't assume DVO enabled means SIL164 on uninitialized chips

It looks like the AST2400 comes up with the DVO enable bit set,
which causes us to incorrectly assume we have a SIL164 regardless
of the value of the scratch registers setup by the BMC firmware.

So let's limit that test to the case where the chip has already
been setup by a BIOS.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/drivers/gpu/drm/ast/ast_main.c b/drivers/gpu/drm/ast/ast_main.c
index 556d065590cc..48998b2102a5 100644
--- a/drivers/gpu/drm/ast/ast_main.c
+++ b/drivers/gpu/drm/ast/ast_main.c
@@ -158,16 +158,22 @@ static int ast_detect_chip(struct drm_device *dev, bool *need_post)
         /*
          * VGACRA3 Enhanced Color Mode Register, check if DVO is already
          * enabled, in that case, assume we have a SIL164 TMDS transmitter
+         *
+         * Don't make that assumption if we the chip wasn't enabled and
+         * is at power-on reset, otherwise we'll incorrectly "detect" a
+         * SIL164 when there is none.
          */
-        jreg = ast_get_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xa3, 0xff);
-        if (jreg & 0x80)
-                ast->tx_chip_type = AST_TX_SIL164;
+        if (!*need_post) {
+                jreg = ast_get_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xa3, 0xff);
+                if (jreg & 0x80)
+                        ast->tx_chip_type = AST_TX_SIL164;
+        }
 
         if ((ast->chip == AST2300) || (ast->chip == AST2400)) {
                 /*
                  * On AST2300 and 2400, look the configuration set by the SoC in
                  * the SOC scratch register #1 bits 11:8 (interestingly marked
-                 * as "reserved" in the spec
+                 * as "reserved" in the spec)
                  */
                 jreg = ast_get_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xd1, 0xff);
                 switch (jreg) {