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

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Date:   Thu, 24 Jun 2021 15:22:23 +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 11:52:47AM +0200, Christian König wrote:
> I've already converted a bunch of the GPU drivers, but there are at least 6 
> GPU still needing to be fixed and on top of that comes VA-API and a few 
> others.
>
> What are your plans for the DMA mapping subsystem?

Building a new API that allows batched DMA mapping without the scatterlist.
The main input for my use case would be bio_vecs, but I plan to make it
a little flexible, and the output would be a list of [dma_addr,len]
tuples, with the API being flexible enough to just return a single
[dma_addr,len] for the common IOMMU coalescing case.

>
>> 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.
>
> Can you give an example?

Take a look at the get_sg_table and put_sg_table helpers in udmabuf.
Those would also be useful in armda, i915, tegra, gntdev-dmabuf, mbochs
in one form or another.

Similar for variants that use a contigous regions.