linux-kernel - Re: [PATCH virtio] virtio: virtio_console: fix DMA memory allocation for rproc serial

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Message-ID: <20201117140230.GA30567@infradead.org>
Date:   Tue, 17 Nov 2020 14:02:30 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Arnaud POULIQUEN <arnaud.pouliquen@...com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Alexander Lobakin <alobakin@...me>,
        Amit Shah <amit@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Suman Anna <s-anna@...com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-remoteproc@...r.kernel.org" <linux-remoteproc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH virtio] virtio: virtio_console: fix DMA memory allocation
 for rproc serial

On Tue, Nov 17, 2020 at 03:00:32PM +0100, Arnaud POULIQUEN wrote:
> 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.

Weird.  I thought virtio was pretty strict in not allowing diret DMA
API usage in drivers to support the legacy no-mapping case.

Either way, the point stands:  if you want these magic buffers handed
out to specific rpmsg instances I think not having to detour through the
DMA API is going to make everyones life easier.