linux-kernel - Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

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Message-ID: <20190116160639.GA28619@lst.de>
Date:   Wed, 16 Jan 2019 17:06:39 +0100
From:   "hch@....de" <hch@....de>
To:     "Koenig, Christian" <Christian.Koenig@....com>
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        "hch@....de" <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yong.zhi@...el.com" <yong.zhi@...el.com>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "bingbu.cao@...el.com" <bingbu.cao@...el.com>,
        "jian.xu.zheng@...el.com" <jian.xu.zheng@...el.com>,
        "tian.shu.qiu@...el.com" <tian.shu.qiu@...el.com>,
        "shiraz.saleem@...el.com" <shiraz.saleem@...el.com>,
        "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "jgg@...pe.ca" <jgg@...pe.ca>
Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

On Wed, Jan 16, 2019 at 07:28:13AM +0000, Koenig, Christian wrote:
> To summarize once more: We have an array of struct pages and want to 
> coherently map that to a device.

And the answer to that is very simple: you can't.  What is so hard
to understand about?  If you want to map arbitrary memory it simply
can't be done in a coherent way on about half of our platforms.

> If that is not possible because of whatever reason we want to get an 
> error code or even not load the driver from the beginning.

That is a bullshit attitude.  Just like everyone else makes their
drivers work you should not be lazy.

> > bool dma_streaming_is_coherent(const struct device *)
> >
> > API to help us decide when to load or not.
> 
> Yes, please.

Hell no.