[REGRESSION] VGA output with AST 2600 graphics.

[Jocelyn Falempe <jfalempe@xxxxxxxxxx>]

Hi,

I've found a regression in the ast driver, for AST2600 hardware.

before the upstream commit f9bd00e0ea9d
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9bd00e0ea9d9b04140aa969a9a13ad3597a1e4e

The ast driver handled AST 2600 chip like an AST 2500.

After this commit, it uses some default values, more like the older AST 
chip.

There are a lot of places in the driver like this:
https://elixir.bootlin.com/linux/v5.18.1/source/drivers/gpu/drm/ast/ast_post.c#L82
where it checks for (AST2300 || AST2400 || AST2500) but not for AST2600.

This makes the VGA output, to be blurred and flickered with whites lines 
on AST2600.

The issue is present since v5.11

For v5.11~v5.17 I propose a simple workaround (as there are no other 
reference to AST2600 in the driver):
--- a/drivers/gpu/drm/ast/ast_main.c
+++ b/drivers/gpu/drm/ast/ast_main.c
@@ -146,7 +146,8 @@ static int ast_detect_chip(struct drm_device *dev, 
bool *need_post)

 	/* Identify chipset */
 	if (pdev->revision >= 0x50) {
-		ast->chip = AST2600;
+		/* Workaround to use the same codepath for AST2600 */
+		ast->chip = AST2500;
 		drm_info(dev, "AST 2600 detected\n");
 	} else if (pdev->revision >= 0x40) {
 		ast->chip = AST2500;

starting from v5.18, there is another reference to AST2600 in the code
https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/ast/ast_main.c#L212

So I think someone with good aspeed knowledge should review all 
locations where there is a test for AST2500, and figure out what should 
be done for AST2600

Thanks,

--

Jocelyn