[PATCH] drm/mgag200: Fix PLL setup for G200_SE_A rev >=4

From: Jocelyn Falempe <jfalempe@redhat.com>
To: gregkh@linuxfoundation.org
Cc: Jocelyn Falempe <jfalempe@redhat.com>,
	stable@vger.kernel.org, Thomas Zimmermann <tzimmermann@suse.de>
Subject: [PATCH] drm/mgag200: Fix PLL setup for G200_SE_A rev >=4
Date: Wed,  4 Jan 2023 17:27:29 +0100	[thread overview]
Message-ID: <20230104162729.424575-1-jfalempe@redhat.com> (raw)
In-Reply-To: <167284297425244@kroah.com>

commit b389286d0234e1edbaf62ed8bc0892a568c33662 upstream.

For G200_SE_A, PLL M setting is wrong, which leads to blank screen,
or "signal out of range" on VGA display.
previous code had "m |= 0x80" which was changed to
m |= ((pixpllcn & BIT(8)) >> 1);

Tested on G200_SE_A rev 42

This line of code was moved to another file with
commit 877507bb954e ("drm/mgag200: Provide per-device callbacks for
PIXPLLC") but can be easily backported before this commit.

v2: * put BIT(7) First to respect MSB-to-LSB (Thomas)
    * Add a comment to explain that this bit must be set (Thomas)

backported to v6.0.17, it also applies cleanly to v5.15.86

Fixes: 2dd040946ecf ("drm/mgag200: Store values (not bits) in struct mgag200_pll_values")
Cc: stable@vger.kernel.org
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013132810.521945-1-jfalempe@redhat.com
---
 drivers/gpu/drm/mgag200/mgag200_pll.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/mgag200/mgag200_pll.c b/drivers/gpu/drm/mgag200/mgag200_pll.c
index 8065ca5d8de9..4f55961848a8 100644
--- a/drivers/gpu/drm/mgag200/mgag200_pll.c
+++ b/drivers/gpu/drm/mgag200/mgag200_pll.c
@@ -269,7 +269,8 @@ static void mgag200_pixpll_update_g200se_04(struct mgag200_pll *pixpll,
 	pixpllcp = pixpllc->p - 1;
 	pixpllcs = pixpllc->s;
 
-	xpixpllcm = pixpllcm | ((pixpllcn & BIT(8)) >> 1);
+	// For G200SE A, BIT(7) should be set unconditionally.
+	xpixpllcm = BIT(7) | pixpllcm;
 	xpixpllcn = pixpllcn;
 	xpixpllcp = (pixpllcs << 3) | pixpllcp;
 
-- 
2.38.1

