linux-kernel - Re: [PATCH] udmabuf: add vmap method to udmabuf

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Date:   Sat, 12 Nov 2022 07:11:08 +0100
From:   Lukasz Wiecaszek <lukasz.wiecaszek@...glemail.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     Christian König <christian.koenig@....com>,
        Lukasz Wiecaszek <lukasz.wiecaszek@...glemail.com>,
        kraxel@...hat.com, Sumit Semwal <sumit.semwal@...aro.org>,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] udmabuf: add vmap method to udmabuf_ops

On Fri, Nov 11, 2022 at 03:31:15PM +0300, Dmitry Osipenko wrote:
> 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
>

Give me some time guys. I need to prepare patch agains 6.1. And this is
my first time, so now it hurts. 

Lukasz