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

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Message-ID: <20230330041726.w7boceq7ljymvfq2@ldmartin-desk2>
Date:   Wed, 29 Mar 2023 22:17:26 -0600
From:   Lucas De Marchi <lucas.demarchi@...el.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
CC:     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>,
        <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment
 clause

On Wed, Mar 29, 2023 at 11:04:17AM +0200, Thomas Zimmermann wrote:
>(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.

using a initializer for the cases it's needed IMO would make these kind
of problems go away, because then the intent is explicit

>
>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

sounds fine to me, but I'd have to take a deeper look later (or when
someone writes the patch).  It seems we'd replicate almost the entire
API to just accomodate the 2 structs.  And the different types will lead
to confusion when one or the other should be used

thanks
Lucas De Marchi

>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