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Subject: Re: [PATCH virtio] virtio: virtio_console: fix DMA memory allocation
 for rproc serial
To:     Christoph Hellwig <hch@infradead.org>
CC:     Alexander Lobakin <alobakin@pm.me>, Amit Shah <amit@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Suman Anna <s-anna@ti.com>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Ohad Ben-Cohen <ohad@wizery.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        "linux-remoteproc@vger.kernel.org" <linux-remoteproc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
References: <AOKowLclCbOCKxyiJ71WeNyuAAj2q8EUtxrXbyky5E@cp7-web-042.plabs.ch>
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 <20201116162844.GB16619@infradead.org> <20201116163907.GA19209@infradead.org>
From:   Arnaud POULIQUEN <arnaud.pouliquen@st.com>
Message-ID: <79d2eb78-caad-9c0d-e130-51e628cedaaa@st.com>
Date:   Tue, 17 Nov 2020 15:00:32 +0100
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On 11/16/20 5:39 PM, Christoph Hellwig wrote:
> Btw, I also still don't understand why remoteproc is using
> dma_declare_coherent_memory to start with.  The virtio code has exactly
> one call to dma_alloc_coherent vring_alloc_queue, a function that
> already switches between two different allocators.  Why can't we just
> add a third allocator specifically for these remoteproc memory carveouts
> and bypass dma_declare_coherent_memory entirely?
> 

The dma_declare_coherent_memory allows to associate vdev0buffer memory region
to the remoteproc virtio device (vdev parent). This region is used to allocated
the rpmsg buffers.
The memory for the rpmsg buffer is allocated by the rpmsg_virtio device in
rpmsg_virtio_bus[1]. The size depends on the total size needed for the rpmsg
buffers.

The vrings are allocated directly by the remoteproc device.

[1]
https://elixir.bootlin.com/linux/v5.10-rc3/source/drivers/rpmsg/virtio_rpmsg_bus.c#L925