From: Daniel Vetter <daniel@ffwll.ch>
To: Volker Vogelhuber <v.vogelhuber@digitalendoscopy.de>
Cc: dri-devel@lists.freedesktop.org
Subject: Re: i915 dma_map_sg return value
Date: Tue, 5 May 2015 17:51:30 +0200 [thread overview]
Message-ID: <20150505155130.GD30184@phenom.ffwll.local> (raw)
In-Reply-To: <55487474.10109@digitalendoscopy.de>
On Tue, May 05, 2015 at 09:42:44AM +0200, Volker Vogelhuber wrote:
> The documentation of the DMA-API writes the following about
> dma_map_sg:
>
> "The implementation is free to merge several consecutive sglist entries
> into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
> consecutive sglist entries can be merged into one provided the first one
> ends and the second one starts on a page boundary - in fact this is a huge
> advantage for cards which either cannot do scatter-gather or have very
> limited number of scatter-gather entries) and returns the actual number
> of sg entries it mapped them to."
>
> I wonder why the return value of dma_map_sg is not returned in any way
> from i915_gem_map_dma_buf. It only uses the return value for error
> checking.
> Can one be sure that in case of the i915 the nents value of the scatter
> gather table is always equal to the value returned by dma_map_sg?
> I'm asking because I want to use the sg table returned by
> i915_gem_map_dma_buf in my own kernel module and iterate over it
> using for_each_sg. And the example in the documentation of the DMA-API
> uses the return value of dma_map_sg when calling for_each_sg and not
> nents and it explicitly mentions:
>
> "Then you should loop count times (note: this can be less than nents times)"
Hm, not looking at the return value of dma_map_sg is also how we use it
internally in i915_gem_gtt.c. Not sure why we get away with this ...
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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next prev parent reply other threads:[~2015-05-05 15:49 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-05 7:42 i915 dma_map_sg return value Volker Vogelhuber
2015-05-05 15:51 ` Daniel Vetter [this message]
2015-05-05 17:19 ` Volker Vogelhuber
2015-05-06 8:24 ` Daniel Vetter
2015-05-06 8:35 ` Chris Wilson
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