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

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Message-ID: <20230330071121.pf4tg4pkwrfv2amc@ldmartin-desk2>
Date:   Thu, 30 Mar 2023 01:11:21 -0600
From:   Lucas De Marchi <lucas.demarchi@...el.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
CC:     suijingfeng <suijingfeng@...ngson.cn>,
        David Airlie <airlied@...ux.ie>, liyi <liyi@...ngson.cn>,
        <linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        Sui Jingfeng <15330273260@....cn>
Subject: Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment
 clause

On Thu, Mar 30, 2023 at 08:57:36AM +0200, 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.

yeah, a split would make it better rather than just make iosys_ptr
replace the current cases where we copy the struct. In this case most
places passing a buffer around in the same driver would migrate to
iosys_ptr.

thanks
Lucas De Marchi

>
>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