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To:     Paul Cercueil <paul@crapouillou.net>,
        Christoph Hellwig <hch@infradead.org>
Cc:     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
References: <20210307202835.253907-1-paul@crapouillou.net>
 <20210307202835.253907-5-paul@crapouillou.net>
 <20210311122846.GC1739082@infradead.org>
 <VJ1TPQ.L5I3WNCQNB982@crapouillou.net>
From:   Thomas Zimmermann <tzimmermann@suse.de>
Subject: Re: [PATCH v2 4/5] drm: Add and export function drm_gem_cma_sync_data
Message-ID: <9c3c8e15-9e8c-4413-e75b-de989a750954@suse.de>
Date:   Mon, 15 Mar 2021 08:43:28 +0100
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From: Thomas Zimmermann <tzimmermann@suse.de>
To: Paul Cercueil <paul@crapouillou.net>,
 Christoph Hellwig <hch@infradead.org>
Cc: 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
Message-ID: <9c3c8e15-9e8c-4413-e75b-de989a750954@suse.de>
Subject: Re: [PATCH v2 4/5] drm: Add and export function drm_gem_cma_sync_data
References: <20210307202835.253907-1-paul@crapouillou.net>
 <20210307202835.253907-5-paul@crapouillou.net>
 <20210311122846.GC1739082@infradead.org>
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In-Reply-To: <VJ1TPQ.L5I3WNCQNB982@crapouillou.net>

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Hi

Am 11.03.21 um 13:33 schrieb Paul Cercueil:
>=20
>=20
> Le jeu. 11 mars 2021 =C3=A0 12:28, Christoph Hellwig <hch@infradead.org=
> a=20
> =C3=A9crit :
>> On Sun, Mar 07, 2021 at 08:28:34PM +0000, Paul Cercueil wrote:
>>> =C2=A0+=C2=A0=C2=A0=C2=A0 drm_atomic_for_each_plane_damage(&iter, &cl=
ip) {
>>> =C2=A0+=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < f=
info->num_planes; i++) {
>>> =C2=A0+=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 daddr =3D drm_fb_cma_get_gem_addr(state->fb, state, i);
>>> =C2=A0+
>>> =C2=A0+=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 /* Ignore x1/x2 values, invalidate complete lines */
>>> =C2=A0+=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 offset =3D clip.y1 * state->fb->pitches[i];
>>> =C2=A0+
>>> =C2=A0+=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 dma_sync_single_for_device(dev, daddr + offset,
>>> =C2=A0+=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 (cl=
ip.y2 - clip.y1) * state->fb->pitches[i],
>>> =C2=A0+=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 DMA=
_TO_DEVICE);
>>
>> Are these helpers only ever used to transfer data to the device and
>> never from it?=C2=A0 If so please clearly document that.
>=20
> 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=20
framebuffer content during blit operations. For devices where this is a=20
slow operation (e.g., PCI read) we set struct=20
drm_mode_config.prefer_shadow to hint renderers to use shadow buffering.

Best regards
Thomas

>=20
> -Paul
>=20
>=20

--=20
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
(HRB 36809, AG N=C3=BCrnberg)
Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer


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