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

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Message-ID: <c4d0facc-7a91-d1a1-d0ce-c5f16c2093a4@189.cn>
Date:   Fri, 31 Mar 2023 11:47:07 +0800
From:   Sui Jingfeng <15330273260@....cn>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Lucas De Marchi <lucas.demarchi@...el.com>
Cc:     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 15:26, Thomas Zimmermann wrote:
> Hi
>
> Am 30.03.23 um 09:17 schrieb Sui Jingfeng:
>> Hi,
>>
>> 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.
>>>
>>   Here i fix a typo, 'tine' -> 'time'
>>
>> As far as i can see, they are two major type of memory in the system.
>>
>> System memory or VRAM,  for the gpu with dedicate video ram, VRAM is 
>> belong to the IO memory category.
>>
>> But there are system choose carveout part of system ram as video 
>> ram(i915?,  for example).
>>
>> the name iosys_map and iosys_ptr have no difference at the first 
>> sight, tell me which one is for mapping system ram
>>
>> and which one is for mapping vram?
>
> As you say correctly, graphics buffers and be in various locations. 
> They can even move between I/O and system memory. The idea behind 
> iosys_map ("I/O and/or system mapping") is that it's a single 
> interface that can handle both.
>
They are all pointers in its very nature.

The hard part to make ensure that  iosys_map can not be replaced with 
iosys_ptr,

They should not overlap in functional, I meant.

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