linux-kernel - Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment clause

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Date:   Wed, 29 Mar 2023 11:04:17 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Sui Jingfeng <15330273260@....cn>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        suijingfeng <suijingfeng@...ngson.cn>, liyi <liyi@...ngson.cn>,
        Lucas De Marchi <lucas.demarchi@...el.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment
 clause

(cc'ing Lucas)

Hi

Am 25.03.23 um 08:46 schrieb Sui Jingfeng:
>   The assignment already done in drm_client_buffer_vmap(),
>   just trival clean, no functional change.
> 
> Signed-off-by: Sui Jingfeng <15330273260@....cn>
> ---
>   drivers/gpu/drm/drm_fbdev_generic.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_fbdev_generic.c b/drivers/gpu/drm/drm_fbdev_generic.c
> index 4d6325e91565..1da48e71c7f1 100644
> --- a/drivers/gpu/drm/drm_fbdev_generic.c
> +++ b/drivers/gpu/drm/drm_fbdev_generic.c
> @@ -282,7 +282,7 @@ static int drm_fbdev_damage_blit(struct drm_fb_helper *fb_helper,
>   				 struct drm_clip_rect *clip)
>   {
>   	struct drm_client_buffer *buffer = fb_helper->buffer;
> -	struct iosys_map map, dst;
> +	struct iosys_map map;
>   	int ret;
>   
>   	/*
> @@ -302,8 +302,7 @@ static int drm_fbdev_damage_blit(struct drm_fb_helper *fb_helper,
>   	if (ret)
>   		goto out;
>   
> -	dst = map;
> -	drm_fbdev_damage_blit_real(fb_helper, clip, &dst);
> +	drm_fbdev_damage_blit_real(fb_helper, clip, &map);

I see what you're doing and it's probably correct in this case.

But there's a larger issue with this iosys interfaces. Sometimes the 
address has to be modified (see calls of iosys_map_incr()). That can 
prevent incorrect uses of the mapping in other places, especially in 
unmap code.

I think it would make sense to consider a separate structure for the I/O 
location. The buffer as a whole would still be represented by struct 
iosys_map.  And that new structure, let's call it struct iosys_ptr, 
would point to an actual location within the buffer's memory range. A 
few locations and helpers would need changes, but there are not so many 
callers that it's an issue.  This would also allow for a few debugging 
tests that ensure that iosys_ptr always operates within the bounds of an 
iosys_map.

I've long considered this idea, but there was no pressure to work on it. 
Maybe now.

Best regards
Thomas

>   
>   	drm_client_buffer_vunmap(buffer);
>   

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

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