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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 16, 2020 at 09:19:50AM +0000, Christoph Hellwig wrote:
> I just noticed this showing up in Linus' tree and I'm not happy.
> 
> This whole model of the DMA subdevices in remoteproc is simply broken.
> 
> We really need to change the virtio code pass an expicit DMA device (
> similar to what e.g. the USB and RDMA code does),

Could you point me at an example or two please?

Thanks!

> instead of faking up
> devices with broken adhoc inheritance of DMA properties and magic poking
> into device parent relationships.
> 
> Bjorn, I thought you were going to look into this a while ago?
> 
> 
> On Wed, Nov 04, 2020 at 03:31:36PM +0000, Alexander Lobakin wrote:
> > Since commit 086d08725d34 ("remoteproc: create vdev subdevice with
> > specific dma memory pool"), every remoteproc has a DMA subdevice
> > ("remoteprocX#vdevYbuffer") for each virtio device, which inherits
> > DMA capabilities from the corresponding platform device. This allowed
> > to associate different DMA pools with each vdev, and required from
> > virtio drivers to perform DMA operations with the parent device
> > (vdev->dev.parent) instead of grandparent (vdev->dev.parent->parent).
> > 
> > virtio_rpmsg_bus was already changed in the same merge cycle with
> > commit d999b622fcfb ("rpmsg: virtio: allocate buffer from parent"),
> > but virtio_console did not. In fact, operations using the grandparent
> > worked fine while the grandparent was the platform device, but since
> > commit c774ad010873 ("remoteproc: Fix and restore the parenting
> > hierarchy for vdev") this was changed, and now the grandparent device
> > is the remoteproc device without any DMA capabilities.
> > So, starting v5.8-rc1 the following warning is observed:
> > 
> > [    2.483925] ------------[ cut here ]------------
> > [    2.489148] WARNING: CPU: 3 PID: 101 at kernel/dma/mapping.c:427 0x80e7eee8
> > [    2.489152] Modules linked in: virtio_console(+)
> > [    2.503737]  virtio_rpmsg_bus rpmsg_core
> > [    2.508903]
> > [    2.528898] <Other modules, stack and call trace here>
> > [    2.913043]
> > [    2.914907] ---[ end trace 93ac8746beab612c ]---
> > [    2.920102] virtio-ports vport1p0: Error allocating inbufs
> > 
> > kernel/dma/mapping.c:427 is:
> > 
> > WARN_ON_ONCE(!dev->coherent_dma_mask);
> > 
> > obviously because the grandparent now is remoteproc dev without any
> > DMA caps:
> > 
> > [    3.104943] Parent: remoteproc0#vdev1buffer, grandparent: remoteproc0
> > 
> > Fix this the same way as it was for virtio_rpmsg_bus, using just the
> > parent device (vdev->dev.parent, "remoteprocX#vdevYbuffer") for DMA
> > operations.
> > This also allows now to reserve DMA pools/buffers for rproc serial
> > via Device Tree.
> > 
> > Fixes: c774ad010873 ("remoteproc: Fix and restore the parenting hierarchy for vdev")
> > Cc: stable@vger.kernel.org # 5.1+
> > Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> > ---
> >  drivers/char/virtio_console.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> > index a2da8f768b94..1836cc56e357 100644
> > --- a/drivers/char/virtio_console.c
> > +++ b/drivers/char/virtio_console.c
> > @@ -435,12 +435,12 @@ static struct port_buffer *alloc_buf(struct virtio_device *vdev, size_t buf_size
> >  		/*
> >  		 * Allocate DMA memory from ancestor. When a virtio
> >  		 * device is created by remoteproc, the DMA memory is
> > -		 * associated with the grandparent device:
> > -		 * vdev => rproc => platform-dev.
> > +		 * associated with the parent device:
> > +		 * virtioY => remoteprocX#vdevYbuffer.
> >  		 */
> > -		if (!vdev->dev.parent || !vdev->dev.parent->parent)
> > +		buf->dev = vdev->dev.parent;
> > +		if (!buf->dev)
> >  			goto free_buf;
> > -		buf->dev = vdev->dev.parent->parent;
> >  
> >  		/* Increase device refcnt to avoid freeing it */
> >  		get_device(buf->dev);
> > -- 
> > 2.29.2
> > 
> > 
> ---end quoted text---

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