linux-kernel - Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

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Message-ID: <20210624081237.GA30289@lst.de>
Date:   Thu, 24 Jun 2021 10:12:37 +0200
From:   Christoph Hellwig <hch@....de>
To:     Christian König <christian.koenig@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Oded Gabbay <oded.gabbay@...il.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König 
        <ckoenig.leichtzumerken@...il.com>,
        Gal Pressman <galpress@...zon.com>, sleybo@...zon.com,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Oded Gabbay <ogabbay@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Doug Ledford <dledford@...hat.com>,
        Tomer Tayar <ttayar@...ana.ai>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to
 export FD for DMA-BUF

On Thu, Jun 24, 2021 at 10:07:14AM +0200, Christian König wrote:
> The key point is that accessing the underlying pages even when DMA-bufs are 
> backed by system memory is illegal. Daniel even created a patch which 
> mangles the page pointers in sg_tables used by DMA-buf to make sure that 
> people don't try to use them.

Which is another goddamn layering violation of a subsystem that has no
business at all poking into the scatterlist structure, yes.

> So the conclusion is that using sg_table in the DMA-buf framework was just 
> the wrong data structure and we should have invented a new one.

I think so.

> But then people would have complained that we have a duplicated 
> infrastructure (which is essentially true).

I doubt it.  At least if you had actually talked to the relevant people.
Which seems to be a major issue with what is going on GPU land.

> My best plan to get out of this mess is that we change the DMA-buf 
> interface to use an array of dma_addresses instead of the sg_table object 
> and I have already been working on this actively the last few month.

Awesome!  I have a bit of related work on the DMA mapping subsystems, so
let's sync up as soon as you have some first sketches.

Btw, one thing I noticed when looking over the dma-buf instances is that
there is a lot of duplicated code for creating a sg_table from pages,
and then mapping it.  It would be good if we could move toward common
helpers instead of duplicating that all over again.