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: <f59ca863-a1c0-0793-99c8-60674da0bfb6@amd.com>
Date:   Tue, 22 Jun 2021 17:49:41 +0200
From:   Christian König <christian.koenig@....com>
To:     Oded Gabbay <oded.gabbay@...il.com>
Cc:     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>,
        Christoph Hellwig <hch@....de>,
        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

Am 22.06.21 um 17:40 schrieb Oded Gabbay:
> On Tue, Jun 22, 2021 at 6:31 PM Christian König
> <christian.koenig@....com> wrote:
>>
>>
>> Am 22.06.21 um 17:28 schrieb Jason Gunthorpe:
>>> On Tue, Jun 22, 2021 at 05:24:08PM +0200, Christian König wrote:
>>>
>>>>>> I will take two GAUDI devices and use one as an exporter and one as an
>>>>>> importer. I want to see that the solution works end-to-end, with real
>>>>>> device DMA from importer to exporter.
>>>>> I can tell you it doesn't. Stuffing physical addresses directly into
>>>>> the sg list doesn't involve any of the IOMMU code so any configuration
>>>>> that requires IOMMU page table setup will not work.
>>>> Sure it does. See amdgpu_vram_mgr_alloc_sgt:
>>>>
>>>>           amdgpu_res_first(res, offset, length, &cursor);
>>>            ^^^^^^^^^^
>>>
>>> I'm not talking about the AMD driver, I'm talking about this patch.
>>>
>>> +             bar_address = hdev->dram_pci_bar_start +
>>> +                             (pages[cur_page] - prop->dram_base_address);
>>> +             sg_dma_address(sg) = bar_address;
>> Yeah, that is indeed not working.
>>
>> Oded you need to use dma_map_resource() for this.
>>
>> Christian.
> Yes, of course.
> But will it be enough ?
> Jason said that supporting IOMMU isn't nice when we don't have struct pages.
> I fail to understand the connection, I need to dig into this.

Question is what you want to do with this?

A struct page is always needed if you want to do stuff like HMM with it, 
if you only want P2P between device I actually recommend to avoid it.

Christian.

>
> Oded
>
>>
>>
>>> Jason