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

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Message-ID: <740bb1d9-fca2-9171-b548-0422c9e94112@189.cn>
Date:   Thu, 30 Mar 2023 17:01:17 +0800
From:   Sui Jingfeng <15330273260@....cn>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Lucas De Marchi <lucas.demarchi@...el.com>
Cc:     suijingfeng <suijingfeng@...ngson.cn>,
        David Airlie <airlied@...ux.ie>, liyi <liyi@...ngson.cn>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment
 clause


On 2023/3/30 14:57, Thomas Zimmermann wrote:
> Hi
>
> Am 30.03.23 um 06:17 schrieb Lucas De Marchi:
>> 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
>
> I think we can split the current interface onto two categories: 
> mapping and I/O. The former would use iosys_map and the latter would 
> use iosys_ptr. And we'd need a helper that turns gets a ptr for a 
> given map.
>
> If I find the tine, I'll probably type up a patch.
>
Then merge this trivial patch first?

It will not prohibit anyone achieve a big goal or releasing a better 
patch in the future.


> Best regards
> Thomas
>
>>
>> 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
>>
>>
>>
>