linux-kernel - Re: remoteproc DMA API abuse status

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Date:   Tue, 29 Jun 2021 12:10:26 +0200
From:   Arnaud POULIQUEN <arnaud.pouliquen@...s.st.com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
CC:     Christoph Hellwig <hch@....de>, Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        <linux-remoteproc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: remoteproc DMA API abuse status

Hi Mathieu

On 6/28/21 10:14 PM, Mathieu Poirier wrote:
> I thought your current work on refactoring the rpmsg_char driver was part of the
> early steps on the way to splitting that patchset up...

No it is de-correlated. Here the point is the vdev0buffer memory region declared
in rproc that we associate to the rproc_virtio device to be able to use is for
RPMsg.

When I have a look this here is the approach we had trying to fix it (this
correspond to my patchset):

The objective is to suppress the dma_declare_coherent_memory usage.

=> In this case the "vdev0buffer" virtio buffer memory pool has do be associated
to the a virtio device by a declaration in device tree. Probably as a subnode of
the remoteproc device node.

&rproc {
	#address-cells = <1>;
	#size-cells = <0>;

+	vdev@0 {
+		memory-region = <&vdev0vring0>,	<&vdev0vring1>, <&vdev0buffer>;
+		compatible = "rproc-virtio";
+		reg = <0>;
+		status = "okay";
+	};
 };

=> a reproc virtio platform driver compatible should be created (based on
remoteproc core and remote_virtio restructuring). The memory region will be in
this case associated in a clean way to the remoteproc virtio device.

=> As consequence we would have 2 module devices the remoteproc and remoteproc
virtio. Both as to be synchronized to ensure that all is ready  before starting
the remote processor => reuse of the proc->subdev mechanism + (component
bind/unbind similar to DRM?)

The last but not the least we probably have to maintain the legacy a while to
let time to move on this new device tree architecture.

Now the question are:
- Is this the good approach or could a simpler patch can fix the issue.
- how to address models with one big memory pool used (TI implementation)

Regards,
Arnaud