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

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Message-ID: <f42d8ab8-c765-2517-7d25-6ce1dea320e8@suse.de>
Date:   Thu, 30 Mar 2023 08:57:36 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     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, Sui Jingfeng <15330273260@....cn>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment
 clause

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.

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
> 
> 
> 

-- 
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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