[PATCH] drm/drm_fb_helper: fix fbdev with sparc64

[Thomas Zimmermann]

Hi

Am 09.07.20 um 21:30 schrieb Sam Ravnborg:
> Mark reported that sparc64 would panic while booting using qemu.
> Mark bisected this to a patch that introduced generic fbdev emulation to
> the bochs DRM driver.
> Mark pointed out that a similar bug was fixed before where
> the sys helpers was replaced by cfb helpers.
> 
> The culprint here is that the framebuffer reside in IO memory which
> requires SPARC ASI_PHYS (physical) loads and stores.
> 
> The current bohcs DRM driver uses a shadow buffer.
> So all copying to the framebuffer happens in
> drm_fb_helper_dirty_blit_real().
> 
> The fix is to replace the memcpy with memcpy_toio() from io.h.
> 
> memcpy_toio() uses writeb() where the original fbdev code
> used sbus_memcpy_toio(). The latter uses sbus_writeb().
> 
> The difference between writeb() and sbus_memcpy_toio() is
> that writeb() writes bytes in little-endian, where sbus_writeb() writes
> bytes in big-endian. As endian does not matter for byte writes they are
> the same. So we can safely use memcpy_toio() here.
> 
> For many architectures memcpy_toio() is a simple memcpy().
> One sideeffect that is unknow is if this has any impact on other
> architectures.
> So far the analysis tells that this change is OK for other arch's.
> but testing would be good.
> 
> Signed-off-by: Sam Ravnborg <sam at ravnborg.org>
> Reported-by: Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk>
> Tested-by: Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk>
> Cc: Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: Gerd Hoffmann <kraxel at redhat.com>
> Cc: "David S. Miller" <davem at davemloft.net>
> Cc: sparclinux at vger.kernel.org

So this actually is a problem in practice. Do you know how userspace
handles this?

For this patch

Acked-by: Thomas Zimmermann <tzimmermann at suse.de>

but I'd like to have someone with more architecture expertise ack this
as well.

Best regards
Thomas

> ---
>  drivers/gpu/drm/drm_fb_helper.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 5609e164805f..4d05b0ab1592 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -399,7 +399,7 @@ static void drm_fb_helper_dirty_blit_real(struct drm_fb_helper *fb_helper,
>  	unsigned int y;
>  
>  	for (y = clip->y1; y < clip->y2; y++) {
> -		memcpy(dst, src, len);
> +		memcpy_toio(dst, src, len);
>  		src += fb->pitches[0];
>  		dst += fb->pitches[0];
>  	}
> 

-- 
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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