From: Robert Jarzmik <robert.jarzmik@free.fr>
To: video4linux-list@redhat.com
Subject: pxa_camera: DMA alignment requirement
Date: Sun, 09 Nov 2008 18:01:37 +0100 [thread overview]
Message-ID: <87prl4x28e.fsf@free.fr> (raw)
Hello,
In the pxa camera driver, the 3 DMA channels used are MMU page aligned. Does
somebody remember where that constraint comes from ?
I'm wondering because the planar YUV format generated is something like this for
a 640 * 480 image :
- Y plane => 640 * 480 bytes = 307200 (and this happens to be a multiple of
pagesize : 307200 = 4096 * 75)
- U plane => 640 * 480 / 2 bytes = 153600 (and this is not a multiple of 4096)
- padding to reach next page : 2048 bytes
- V plane => 153600 bytes
- padding to reach next page : 2048 bytes
This means a user space library should be kernel pagesize aware to transform the
output image. I don't really understand the necessity of page aligned DMA
channels. Would someone tell me please ?
--
Robert
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next reply other threads:[~2008-11-09 17:01 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-11-09 17:01 Robert Jarzmik [this message]
2008-11-13 10:23 ` pxa_camera: DMA alignment requirement Mike Rapoport
2008-11-16 1:14 ` Robert Jarzmik
2008-11-16 7:30 ` Mike Rapoport
2008-11-25 21:18 ` Robert Jarzmik
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