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Date:   Mon, 15 Mar 2021 11:59:50 +0000
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH v2 4/5] drm: Add and export function drm_gem_cma_sync_data
To:     Thomas Zimmermann <tzimmermann@suse.de>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sam Ravnborg <sam@ravnborg.org>, od@zcrc.me,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        linux-mips@vger.kernel.org
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Hi Thomas,

Le lun. 15 mars 2021 ? 8:43, Thomas Zimmermann <tzimmermann@suse.de> a 
?crit :
> Hi
> 
> Am 11.03.21 um 13:33 schrieb Paul Cercueil:
>> 
>> 
>> Le jeu. 11 mars 2021 ? 12:28, Christoph Hellwig <hch@infradead.org> 
>> a ?crit :
>>> On Sun, Mar 07, 2021 at 08:28:34PM +0000, Paul Cercueil wrote:
>>>>  +    drm_atomic_for_each_plane_damage(&iter, &clip) {
>>>>  +        for (i = 0; i < finfo->num_planes; i++) {
>>>>  +            daddr = drm_fb_cma_get_gem_addr(state->fb, state, i);
>>>>  +
>>>>  +            /* Ignore x1/x2 values, invalidate complete lines */
>>>>  +            offset = clip.y1 * state->fb->pitches[i];
>>>>  +
>>>>  +            dma_sync_single_for_device(dev, daddr + offset,
>>>>  +                       (clip.y2 - clip.y1) * 
>>>> state->fb->pitches[i],
>>>>  +                       DMA_TO_DEVICE);
>>> 
>>> Are these helpers only ever used to transfer data to the device and
>>> never from it?  If so please clearly document that.
>> 
>> Yes. In the DRM world, are there cases where we transfer data from 
>> the device? I assume these cases are handled by v4l2 instead.
> 
> Software rendering (i.e., anything wrt dumb buffers) likely reads 
> back framebuffer content during blit operations. For devices where 
> this is a slow operation (e.g., PCI read) we set struct 
> drm_mode_config.prefer_shadow to hint renderers to use shadow 
> buffering.

This has been brought up a few times already. I answered that in the 
cover letter. In my case, *writes* (e.g. dumb memcpy) are also slower 
with a write-combine buffer than with a non-coherent buffer + cache 
sync. So a shadow buffer does nothing for me.

Cheers,
-Paul