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From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Sakari Ailus <sakari.ailus@iki.fi>
Cc: Michael Jones <michael.jones@matrix-vision.de>,
	Linux Media Mailing List <linux-media@vger.kernel.org>
Subject: Re: omap3isp buffer alignment
Date: Fri, 12 Aug 2011 11:28:21 +0200	[thread overview]
Message-ID: <201108121128.22850.laurent.pinchart@ideasonboard.com> (raw)
In-Reply-To: <20110811212145.GK5926@valkosipuli.localdomain>

On Thursday 11 August 2011 23:21:45 Sakari Ailus wrote:
> On Thu, Aug 11, 2011 at 11:57:04AM +0200, Michael Jones wrote:
> > Hi Laurent,
> > 
> > If I understood your discussion with Russell [1] correctly, user pointer
> > buffers are required to be page-aligned because of the IOMMU API, and
> > it's desirable to keep the IOMMU driver that way for other subsystems
> > which may use it. So we're stuck with user buffers needing to be
> > page-aligned.
> 
> My understanding is that this is actually a hardware requirement. You only
> can map pages of 4 kiB (at least).

The IOMMU works on a 4kiB page granularity, but that doesn't require buffers 
to be page-aligned. If the buffer start address isn't aligned on a page 
boundary the IOMMU can map the whole page, and the driver can then just add an 
offset to the virtual address.

> > There's a check in ispvideo.c:isp_video_buffer_prepare() that the buffer
> > address is 32-byte aligned. Isn't this superfluous considering the
> > page-aligned restriction?
> 
> I guess the ISP driver isn't assuming that ispmmu_vmap always give page
> aligned mappings --- or that the page size couls theoretically be smaller.
> The assumptions might not hold in another implementation of the IOMMU API,
> which however will be replaced (hopefully at some point) by the improved
> DMA mapping API.

-- 
Regards,

Laurent Pinchart

      reply	other threads:[~2011-08-12  9:28 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-08-11  9:57 omap3isp buffer alignment Michael Jones
2011-08-11 21:21 ` Sakari Ailus
2011-08-12  9:28   ` Laurent Pinchart [this message]

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