linux-kernel - Re: [PATCH v2] drm/ast: Fix soft lockup

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Date: Wed, 3 Apr 2024 11:32:49 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Jammy Huang <jammy_huang@...eedtech.com>, jfalempe@...hat.com,
 maarten.lankhorst@...ux.intel.com, mripard@...nel.org, airlied@...hat.com,
 airlied@...il.com, daniel@...ll.ch
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] drm/ast: Fix soft lockup



Am 03.04.24 um 11:02 schrieb Jammy Huang:
> There is a while-loop in ast_dp_set_on_off() that could lead to
> infinite-loop. This is because the register, VGACRI-Dx, checked in
> this API is a scratch register actually controlled by a MCU, named
> DPMCU, in BMC.
>
> These scratch registers are protected by scu-lock. If suc-lock is not
> off, DPMCU can not update these registers and then host will have soft
> lockup due to never updated status.
>
> DPMCU is used to control DP and relative registers to handshake with
> host's VGA driver. Even the most time-consuming task, DP's link
> training, is less than 100ms. 200ms should be enough.
>
> Signed-off-by: Jammy Huang <jammy_huang@...eedtech.com>
> Reviewed-by: Jocelyn Falempe <jfalempe@...hat.com>

Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>

> ---
>   drivers/gpu/drm/ast/ast_dp.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
> index ebb6d8ebd44e..1e9259416980 100644
> --- a/drivers/gpu/drm/ast/ast_dp.c
> +++ b/drivers/gpu/drm/ast/ast_dp.c
> @@ -180,6 +180,7 @@ void ast_dp_set_on_off(struct drm_device *dev, bool on)
>   {
>   	struct ast_device *ast = to_ast_device(dev);
>   	u8 video_on_off = on;
> +	u32 i = 0;
>   
>   	// Video On/Off
>   	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xE3, (u8) ~AST_DP_VIDEO_ENABLE, on);
> @@ -192,6 +193,8 @@ void ast_dp_set_on_off(struct drm_device *dev, bool on)
>   						ASTDP_MIRROR_VIDEO_ENABLE) != video_on_off) {
>   			// wait 1 ms
>   			mdelay(1);
> +			if (++i > 200)
> +				break;
>   		}
>   	}
>   }
>
> base-commit: b0546776ad3f332e215cebc0b063ba4351971cca

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)