[PATCH] drm/bochs: Add screen blanking support

[Thomas Zimmermann]

Hi

Am 20.04.21 um 18:56 schrieb Takashi Iwai:
> On bochs DRM driver, the execution of "setterm --blank force" results
> in a frozen screen instead of a blank screen.  It's due to the lack of
> the screen blanking support in its code.
> 
> Actually, the QEMU bochs vga side can switch to the blanking mode when
> the bit 0x20 is cleared on VGA_ATT_IW register (0x3c0), which updates
> ar_index in QEMU side.  So, essentially, we'd just need to clear the
> bit at pipe disable callback; that's what this patch does essentially.
> 
> However, a tricky part is that the QEMU vga code does treat VGA_ATT_IW
> register always as "flip-flop"; the first write is for index and the
> second write is for the data like palette.  Meanwhile, in the current
> bochs DRM driver, the flip-flop wasn't considered, and it calls only
> the register update once with the value 0x20.
> 

Unless bochs does things very different, the index should first be reset 
by reading 0x3da. Then write the index, then the data.

https://web.stanford.edu/class/cs140/projects/pintos/specs/freevga/vga/vgareg.htm#attribute

Best regards
Thomas

> So, in this patch, we fix the behavior by simply writing the
> VGA_ATT_IW register value twice at each place as well.
> 
> Signed-off-by: Takashi Iwai <tiwai at suse.de>
> ---
>   drivers/gpu/drm/bochs/bochs_hw.c  | 13 ++++++++++++-
>   drivers/gpu/drm/bochs/bochs_kms.c |  8 ++++++++
>   2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bochs/bochs_hw.c b/drivers/gpu/drm/bochs/bochs_hw.c
> index 2d7380a9890e..9a6f90216d6c 100644
> --- a/drivers/gpu/drm/bochs/bochs_hw.c
> +++ b/drivers/gpu/drm/bochs/bochs_hw.c
> @@ -213,6 +213,14 @@ void bochs_hw_setmode(struct bochs_device *bochs,
>   	if (!drm_dev_enter(bochs->dev, &idx))
>   		return;
>   
> +	if (!mode) {
> +		DRM_DEBUG_DRIVER("crtc disabled\n");
> +		/* set to blank mode; send twice for ar_flip_flop */
> +		bochs_vga_writeb(bochs, 0x3c0, 0);
> +		bochs_vga_writeb(bochs, 0x3c0, 0);
> +		goto exit;
> +	}
> +
>   	bochs->xres = mode->hdisplay;
>   	bochs->yres = mode->vdisplay;
>   	bochs->bpp = 32;
> @@ -223,7 +231,9 @@ void bochs_hw_setmode(struct bochs_device *bochs,
>   			 bochs->xres, bochs->yres, bochs->bpp,
>   			 bochs->yres_virtual);
>   
> -	bochs_vga_writeb(bochs, 0x3c0, 0x20); /* unblank */
> +	/* unblank; send twice for ar_flip_flop */
> +	bochs_vga_writeb(bochs, 0x3c0, 0x20);
> +	bochs_vga_writeb(bochs, 0x3c0, 0x20);
>   
>   	bochs_dispi_write(bochs, VBE_DISPI_INDEX_ENABLE,      0);
>   	bochs_dispi_write(bochs, VBE_DISPI_INDEX_BPP,         bochs->bpp);
> @@ -239,6 +249,7 @@ void bochs_hw_setmode(struct bochs_device *bochs,
>   	bochs_dispi_write(bochs, VBE_DISPI_INDEX_ENABLE,
>   			  VBE_DISPI_ENABLED | VBE_DISPI_LFB_ENABLED);
>   
> + exit:
>   	drm_dev_exit(idx);
>   }
>   
> diff --git a/drivers/gpu/drm/bochs/bochs_kms.c b/drivers/gpu/drm/bochs/bochs_kms.c
> index 853081d186d5..b0d77d6d3ae4 100644
> --- a/drivers/gpu/drm/bochs/bochs_kms.c
> +++ b/drivers/gpu/drm/bochs/bochs_kms.c
> @@ -57,6 +57,13 @@ static void bochs_pipe_enable(struct drm_simple_display_pipe *pipe,
>   	bochs_plane_update(bochs, plane_state);
>   }
>   
> +static void bochs_pipe_disable(struct drm_simple_display_pipe *pipe)
> +{
> +	struct bochs_device *bochs = pipe->crtc.dev->dev_private;
> +
> +	bochs_hw_setmode(bochs, NULL);
> +}
> +
>   static void bochs_pipe_update(struct drm_simple_display_pipe *pipe,
>   			      struct drm_plane_state *old_state)
>   {
> @@ -67,6 +74,7 @@ static void bochs_pipe_update(struct drm_simple_display_pipe *pipe,
>   
>   static const struct drm_simple_display_pipe_funcs bochs_pipe_funcs = 
{
>   	.enable	    = bochs_pipe_enable,
> +	.disable    = bochs_pipe_disable,
>   	.update	    = bochs_pipe_update,
>   	.prepare_fb = drm_gem_vram_simple_display_pipe_prepare_fb,
>   	.cleanup_fb = drm_gem_vram_simple_display_pipe_cleanup_fb,
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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