linux-kernel - Re: [PATCH v2 4/5] drm: Add and export function drm_gem_cma_sync

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Date:   Mon, 15 Mar 2021 08:43:28 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Paul Cercueil <paul@...pouillou.net>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Sam Ravnborg <sam@...nborg.org>, od@...c.me,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH v2 4/5] drm: Add and export function drm_gem_cma_sync_data

Hi

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