Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <512e97ec-5d5e-4d6a-e547-13ca4036f3d1@collabora.com>
Date:   Fri, 11 Nov 2022 15:31:15 +0300
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.3.1
Subject: Re: [PATCH] udmabuf: add vmap method to udmabuf_ops
Content-Language: en-US
To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Lukasz Wiecaszek <lukasz.wiecaszek@googlemail.com>,
        kraxel@redhat.com
Cc:     Lukasz Wiecaszek <lukasz.wiecaszek@gmail.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org,
        linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org
References: <20221111114528.608801-1-lukasz.wiecaszek@gmail.com>
 <2ee10e0e-a347-71a5-051a-02b9bac0bbb6@amd.com>
From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
In-Reply-To: <2ee10e0e-a347-71a5-051a-02b9bac0bbb6@amd.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 11/11/22 15:05, Christian König wrote:
> Adding Dmitry as well.
> 
> Am 11.11.22 um 12:45 schrieb Lukasz Wiecaszek:
>> The reason behind that patch is associated with videobuf2 subsystem
>> (or more genrally with v4l2 framework) and user created
>> dma buffers (udmabuf). In some circumstances
>> when dealing with V4L2_MEMORY_DMABUF buffers videobuf2 subsystem
>> wants to use dma_buf_vmap() method on the attached dma buffer.
>> As udmabuf does not have .vmap operation implemented,
>> such dma_buf_vmap() natually fails.
>>
>> videobuf2_common: [cap-000000003473b2f1] __vb2_queue_alloc: allocated
>> 3 buffers, 1 plane(s) each
>> videobuf2_common: [cap-000000003473b2f1] __prepare_dmabuf: buffer for
>> plane 0 changed
>> videobuf2_common: [cap-000000003473b2f1] __prepare_dmabuf: failed to
>> map dmabuf for plane 0
>> videobuf2_common: [cap-000000003473b2f1] __buf_prepare: buffer
>> preparation failed: -14
>>
>> The patch itself seems to be strighforward.
>> It adds implementation of .vmap method to 'struct dma_buf_ops
>> udmabuf_ops'.
>> .vmap method itself uses vm_map_ram() to map pages linearly
>> into the kernel virtual address space (only if such mapping
>> hasn't been created yet).
> 
> Of hand that sounds sane to me.
> 
> You should probably mention somewhere in a code comment that the cached
> vaddr is protected by the reservation lock being taken. That's not
> necessary obvious to everybody.
> 
> Apart from that looks good to me.

Adding a comment won't hurt.

We have the dma_resv_assert_held() in dma_buf_vmap() that will help
spotting a missing lock at runtime by developers. While the
dmbuf_ops->vmap() shouldn't be ever used directly by importers.

-- 
Best regards,
Dmitry