linux-kernel - Re: [RFC][PATCH] add dma_reserve_coherent_memory()/dma_free_reserved

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Message-Id: <20100826184231J.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 26 Aug 2010 18:43:22 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	mitov@...p.bas.bg
Cc:	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [RFC][PATCH] add
 dma_reserve_coherent_memory()/dma_free_reserved_memory() API

On Thu, 26 Aug 2010 10:01:52 +0300
Marin Mitov <mitov@...p.bas.bg> wrote:

> > If you add something to the videobuf-dma-contig API, that's fine by me
> > because drivers/media/video/videobuf-dma-contig.c uses the own
> > structure and plays with dma_alloc_coherent. As long as a driver
> > doesn't touch device->dma_mem directly, it's fine, 
> 
> Why, my understanding is that device->dma_mem is designed exactly for keeping 
> some chunk of coherent memory for device's private use via dma_alloc_from_coherent()
> (and that is what dt3155v4l driver is using it for).

I don't think so. device->dma_mem can be accessed only via the
DMA-API. I think that the DMA-API says that
dma_declare_coherent_memory declares coherent memory that can be
access exclusively by a certain device. It's not for reserving
coherent memory that can be used for any device for a device.

Anway, you don't need coherent memory. So using the API for coherent
memory isn't a good idea.


> > There are already some workarounds for
> > contigous memory in several drivers anyway.
> 
> Sure, can these workarounds be exposed as API for general use?

I don't think that's a good idea. Adding temporary workaround to the
generic API and removing it soon after that doesn't sound a good
developing maner.
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