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

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Date:   Wed, 18 Nov 2020 05:16:15 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Arnaud POULIQUEN <arnaud.pouliquen@...com>,
        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>,
        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 02:02:30PM +0000, Christoph Hellwig wrote:
> 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.

Well remoteproc is weird in that it's use of DMA API precedes
standartization efforts, and it was never standardized in the virtio
spec ..

> 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.