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Date:   Thu, 24 Jan 2019 17:33:21 +0000
From:   Mark Brown <broonie@kernel.org>
To:     Jaroslav Kysela <perex@perex.cz>
Cc:     Leo Yan <leo.yan@linaro.org>, Takashi Iwai <tiwai@suse.de>,
        alsa-devel@alsa-project.org, arnd@arndb.de, keescook@chromium.org,
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        mchehab+samsung@kernel.org, sboyd@kernel.org, vkoul@kernel.org,
        Baolin Wang <baolin.wang@linaro.org>,
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        o-takashi@sakamocchi.jp, colyli@suse.de,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] ALSA: core: Add DMA share buffer support
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On Thu, Jan 24, 2019 at 02:43:02PM +0100, Jaroslav Kysela wrote:

> If I look to the dma_buf_fd() implementation:

>   fd = get_unused_fd_flags(flags);
>   fd_install(fd, dmabuf->file);

> .. what if we just add one new ioctl to the ALSA's PCM API which will
> return a new anonymous inode descriptor with the restricted access to
> the main PCM device to satisfy the SELinux requirements / security
> policies? It might be more nice and simple solution than to implement
> the full dma-buf interface for the ALSA's PCM devices.

That certainly works for me so long as the security people are happy.

> Question: The dma-buf also implements the fencing, but I am not able to
> determine, if this mechanism is used in android [1]. It may allow
> concurrent mmap and synchronize apps - but the sound server should
> manage the access to the DMA buffer anyway. In my opinion, it makes much
> sense for the video-pipes when the hardware does some accelerations
> (encoding/decoding).

We had the same discuission off list and couldn't think of a need for
that feature in the audio context but left it in as it's already there
with dma-buf so there's no real cost to implementing it and we weren't
sure we weren't missing something.

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