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

[Arnaud POULIQUEN <arnaud.pouliquen@st.com>]

Hi all,

On 11/16/20 10:19 AM, 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), instead of faking up
> devices with broken adhoc inheritance of DMA properties and magic poking
> into device parent relationships.

For your formation I started some stuff on my side to be able to declare the
virtio device in DT as a remoteproc child node.

https://lkml.org/lkml/2020/4/16/1817

Quite big refactoring, but could be a way to answer...

Regards,
Arnaud

> 
> 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---
>