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

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Date:   Mon, 16 Nov 2020 16:30:22 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Alexander Lobakin <alobakin@...me>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Amit Shah <amit@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnaud Pouliquen <arnaud.pouliquen@...com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-remoteproc@...r.kernel.org, Suman Anna <s-anna@...com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH virtio] virtio: virtio_console: fix DMA memory allocation
 for rproc serial

On Mon, Nov 16, 2020 at 01:07:28PM +0000, Alexander Lobakin wrote:
> But lots of subsystems like netdev for example uses dev->parent for
> DMA operations. I know that their pointers go directly to the
> platform/PCI/etc. device, but still.

Oh, every drivers is perfectly fine to use ->parent as it suits.  The
problem is when we have layered architectures, where this pokes a
massive hole into the layering.

> The only reason behind "fake" DMA devices for rproc is to be able to
> reserve DMA memory through the Device Tree exclusively for only one
> virtio dev like virtio_console or virtio_rpmsg_bus. That's why
> they are present, are coercing DMA caps from physical dev
> representor, and why questinable dma_declare_coherent_memory()
> is still here and doesn't allow to build rproc core as a module.
> I agree that this is not the best model obviously, and we should take
> a look at it.

As far as I can tell the series from Arnaud does the right thing here.