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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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             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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