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

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Date:   Wed, 29 Mar 2023 11:09:13 +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



Am 29.03.23 um 11:04 schrieb Thomas Zimmermann:
> (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 

'prevent correct uses'

> 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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