Re: [PATCH 2/3] rpmsg: core: make rpmsg bus DMA capable

[Bjorn Andersson]

On Fri 02 Mar 08:14 PST 2018, Robin Murphy wrote:

> On 02/03/18 14:55, srinivas.kandaga...@linaro.org wrote:
> > From: Srinivas Kandagatla 
> > 
> > Many of the rpmsg clients like audio drivers need to allocate
> > dma memory. Make this bus DMA capable so that the child devices
> > can use dma apis.
> 
> AFAICS after 15 minutes in the docs and code, the rpmsg "bus" is a virtual
> one based around shared-memory mailbox communication, so I don't really see
> how DMA exists in that context - I think maybe that abstraction needs
> looking at.
> 

That's right, rpmsg shuffles messages back and forth to some coprocessor
over shared memory, the contexts generating and receiving these messages
are "rpmsg devices".

The problem Srinivas is facing is that one of these rpmsg devices is
trying to allocate and map a larger chunk of memory to be shared with
the coprocessor, which is then going to be referenced in the messages
being passed in rpmsg.

> However, from grepping through the DTs it seems at first glance like the
> non-trivial things under the "qcom,smd" bus mostly map to actual platform
> devices via the "qcom,smd-edge" property - if those platform devices are the
> physical DMA masters, they should be the ones used for DMA API operations.
> 

One of the rpmsg implementations is virtio based and have a similar
problem, there dma_alloc*() is called with dev->parent->parent as
device, but this causes issues as dev->parent might not be what the
original author expected it to -- so this needs to be reworked as well.

> > Signed-off-by: Srinivas Kandagatla 
> > ---
> >   drivers/rpmsg/rpmsg_core.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/rpmsg/rpmsg_core.c b/drivers/rpmsg/rpmsg_core.c
> > index e84c71f8d6ab..540a3f3567b8 100644
> > --- a/drivers/rpmsg/rpmsg_core.c
> > +++ b/drivers/rpmsg/rpmsg_core.c
> > @@ -472,6 +472,7 @@ struct bus_type rpmsg_bus = {
> > .uevent = rpmsg_uevent,
> > .probe  = rpmsg_dev_probe,
> > .remove = rpmsg_dev_remove,
> > +   .force_dma  = true,
> 
> Regardless of the above, would you really need to use this brute force hack
> instead of just fixing the DTs? I'm struggling to find which drivers might
> currently be relying on this :/
> 

The rpmsg devices, described as child nodes of rpmsg bus relates to
specific functions in the coprocessor firmware. The fact that the
firmware can be started and stopped dynamically makes the current layout
quite convenient (in comparison to e.g. how we would describe a
mailbox).

We know for these cases that dev->parent->parent is a remoteproc
instance representing the coprocessor that sits on the other side of the
communication channel. So we did investigate if we could just have that
to allocate and map buffers. The problem with this is that these
functions has multiple iommu contexts.

Regards,
Bjorn
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Re: [PATCH 2/3] rpmsg: core: make rpmsg bus DMA capable

[Srinivas Kandagatla]

Thanks for your time,

On 02/03/18 16:14, Robin Murphy wrote:

On 02/03/18 14:55, srinivas.kandaga...@linaro.org wrote:

From: Srinivas Kandagatla 

Many of the rpmsg clients like audio drivers need to allocate
dma memory. Make this bus DMA capable so that the child devices
can use dma apis.


AFAICS after 15 minutes in the docs and code, the rpmsg "bus" is a 
virtual one based around shared-memory mailbox communication, so I don't 
really see how DMA exists in that context - I think maybe that 
abstraction needs looking at.


However, from grepping through the DTs it seems at first glance like the 
non-trivial things under the "qcom,smd" bus mostly map to actual 
platform devices via the "qcom,smd-edge" property - if those platform 
devices are the physical DMA masters, they should be the ones used for 
DMA API operations.


Currently there are very limited rpmsg devices in the mainline that use 
dma. Only one I can think of is wcnss WIFI driver which models up itself 
into another layer of platform device. Not sure if the DMA was the 
reason to do that.


However am working on audio drivers [1] which I modeled up as children 
of the rpmsg bus, so the problem started. There is an IOMMU in between 
APPs and DSP which provides audio services.
There are also other projects like FastRPC which have used similar 
driver model which ended up with same issues.


It all depends on how you model your driver. Audio case we have a rpmsg 
channel which exposes audio functionality. so If we want to use the 
iommu/dma operations we have to add another layer of platform device.
Which also means that rpmsg channel notifications have to be passed to 
these platform devices in some way.


Am not 100% sure if this correct way to fix the issue.




Signed-off-by: Srinivas Kandagatla 
---
  drivers/rpmsg/rpmsg_core.c | 1 +
  1 file changed, 1 insertion(+)

diff --git a/drivers/rpmsg/rpmsg_core.c b/drivers/rpmsg/rpmsg_core.c
index e84c71f8d6ab..540a3f3567b8 100644
--- a/drivers/rpmsg/rpmsg_core.c
+++ b/drivers/rpmsg/rpmsg_core.c
@@ -472,6 +472,7 @@ struct bus_type rpmsg_bus = {
  .uevent= rpmsg_uevent,
  .probe= rpmsg_dev_probe,
  .remove= rpmsg_dev_remove,
+.force_dma= true,


Regardless of the above, would you really need to use this brute force 
hack instead of just fixing the DTs? I'm struggling to find which 
drivers might currently be relying on this :/


This is one of the two issues. dma-ranges might work in this case, but 
we still have iommu case.




Robin.


  };
  EXPORT_SYMBOL(rpmsg_bus);



thanks,
srini
[1]: https://lkml.org/lkml/2018/2/13/719
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Re: [PATCH 2/3] rpmsg: core: make rpmsg bus DMA capable

[Robin Murphy]

On 02/03/18 14:55, srinivas.kandaga...@linaro.org wrote:

From: Srinivas Kandagatla 

Many of the rpmsg clients like audio drivers need to allocate
dma memory. Make this bus DMA capable so that the child devices
can use dma apis.


AFAICS after 15 minutes in the docs and code, the rpmsg "bus" is a 
virtual one based around shared-memory mailbox communication, so I don't 
really see how DMA exists in that context - I think maybe that 
abstraction needs looking at.


However, from grepping through the DTs it seems at first glance like the 
non-trivial things under the "qcom,smd" bus mostly map to actual 
platform devices via the "qcom,smd-edge" property - if those platform 
devices are the physical DMA masters, they should be the ones used for 
DMA API operations.



Signed-off-by: Srinivas Kandagatla 
---
  drivers/rpmsg/rpmsg_core.c | 1 +
  1 file changed, 1 insertion(+)

diff --git a/drivers/rpmsg/rpmsg_core.c b/drivers/rpmsg/rpmsg_core.c
index e84c71f8d6ab..540a3f3567b8 100644
--- a/drivers/rpmsg/rpmsg_core.c
+++ b/drivers/rpmsg/rpmsg_core.c
@@ -472,6 +472,7 @@ struct bus_type rpmsg_bus = {
.uevent = rpmsg_uevent,
.probe  = rpmsg_dev_probe,
.remove = rpmsg_dev_remove,
+   .force_dma  = true,


Regardless of the above, would you really need to use this brute force 
hack instead of just fixing the DTs? I'm struggling to find which 
drivers might currently be relying on this :/


Robin.


  };
  EXPORT_SYMBOL(rpmsg_bus);
  


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