linux-kernel - Re: [PATCH v4 2/2] habanalabs: add support for dma-buf exporter

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Date:   Tue, 6 Jul 2021 10:54:51 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Oded Gabbay <oded.gabbay@...il.com>
Cc:     Oded Gabbay <ogabbay@...nel.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Gal Pressman <galpress@...zon.com>, sleybo@...zon.com,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Doug Ledford <dledford@...hat.com>,
        Dave Airlie <airlied@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        Christoph Hellwig <hch@....de>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, Tomer Tayar <ttayar@...ana.ai>
Subject: Re: [PATCH v4 2/2] habanalabs: add support for dma-buf exporter

On Tue, Jul 06, 2021 at 12:44:49PM +0300, Oded Gabbay wrote:

> > > +     /* In case we got a large memory area to export, we need to divide it
> > > +      * to smaller areas because each entry in the dmabuf sgt can only
> > > +      * describe unsigned int.
> > > +      */
> >
> > Huh? This is forming a SGL, it should follow the SGL rules which means
> > you have to fragment based on the dma_get_max_seg_size() of the
> > importer device.
> >
> hmm
> I don't see anyone in drm checking this value (and using it) when
> creating the SGL when exporting dmabuf. (e.g.
> amdgpu_vram_mgr_alloc_sgt)

For dmabuf the only importer is RDMA and it doesn't care, but you
certainly should not introduce a hardwired constant instead of using
the correct function.

Jason