Re: [PATCH v2 4/5] drm: Add and export function drm_gem_cma_sync_data

[Thomas Zimmermann]

Hi

Am 11.03.21 um 13:33 schrieb Paul Cercueil:
> Le jeu. 11 mars 2021 à 12:28, Christoph Hellwig <hch@xxxxxxxxxxxxx> a 
> écrit :
> > 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.

Best regards
Thomas

> -Paul

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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