Linux Virtualization - Feb 2020 - [PATCH 5/5] vdpasim: vDPA device simulator

On 2020/2/4 ??4:21, Zhu Lingshan wrote:
>> +static const struct dma_map_ops vdpasim_dma_ops = {
>> +??? .map_page = vdpasim_map_page,
>> +??? .unmap_page = vdpasim_unmap_page,
>> +??? .alloc = vdpasim_alloc_coherent,
>> +??? .free = vdpasim_free_coherent,
>> +};
>> +
>
> Hey Jason,
>
> IMHO, it would be nice if dma_ops of the parent device could be 
> re-used. vdpa_device is expecting to represent a physical device 
> except this simulator, however, there are not enough information in 
> vdpa_device.dev to indicating which kind physical device it attached 
> to. Namely get_arch_dma_ops(struct bus type) can not work on 
> vdpa_device.dev. Then it seems device drivers need to implement a wrap 
> of dma_ops of parent devices. Can this work be done in the vdpa 
> framework since it looks like a common task? Can 
> "vd_dev->vdev.dev.parent = vdpa->dev->parent;" in
virtio_vdpa_probe()
> do the work?
>
> Thanks,
> BR
> Zhu Lingshan 

Good catch.

I think we can.

Thanks

On Tue, Feb 04, 2020 at 04:28:27PM +0800, Jason Wang
wrote:
> 
> On 2020/2/4 ??4:21, Zhu Lingshan wrote:
> > > +static const struct dma_map_ops vdpasim_dma_ops = {
> > > +??? .map_page = vdpasim_map_page,
> > > +??? .unmap_page = vdpasim_unmap_page,
> > > +??? .alloc = vdpasim_alloc_coherent,
> > > +??? .free = vdpasim_free_coherent,
> > > +};
> > > +
> > 
> > Hey Jason,
> > 
> > IMHO, it would be nice if dma_ops of the parent device could be
re-used.
> > vdpa_device is expecting to represent a physical device except this
> > simulator, however, there are not enough information in
vdpa_device.dev
> > to indicating which kind physical device it attached to. Namely
> > get_arch_dma_ops(struct bus type) can not work on vdpa_device.dev.
Then
> > it seems device drivers need to implement a wrap of dma_ops of parent
> > devices. Can this work be done in the vdpa framework since it looks
like
> > a common task? Can "vd_dev->vdev.dev.parent =
vdpa->dev->parent;" in
> > virtio_vdpa_probe() do the work?
> > 
> > Thanks,
> > BR
> > Zhu Lingshan
> 
> 
> Good catch.
> 
> I think we can.
IMHO you need to specify some 'dma_device', not try and play tricks
with dma_ops, or assuming the parent is always the device used for
dma.

Jason

On 2020/2/4 ??8:52, Jason Gunthorpe wrote:
> On Tue, Feb 04, 2020 at 04:28:27PM +0800, Jason Wang wrote:
>> On 2020/2/4 ??4:21, Zhu Lingshan wrote:
>>>> +static const struct dma_map_ops vdpasim_dma_ops = {
>>>> +??? .map_page = vdpasim_map_page,
>>>> +??? .unmap_page = vdpasim_unmap_page,
>>>> +??? .alloc = vdpasim_alloc_coherent,
>>>> +??? .free = vdpasim_free_coherent,
>>>> +};
>>>> +
>>> Hey Jason,
>>>
>>> IMHO, it would be nice if dma_ops of the parent device could be
re-used.
>>> vdpa_device is expecting to represent a physical device except this
>>> simulator, however, there are not enough information in
vdpa_device.dev
>>> to indicating which kind physical device it attached to. Namely
>>> get_arch_dma_ops(struct bus type) can not work on vdpa_device.dev.
Then
>>> it seems device drivers need to implement a wrap of dma_ops of
parent
>>> devices. Can this work be done in the vdpa framework since it looks
like
>>> a common task? Can "vd_dev->vdev.dev.parent =
vdpa->dev->parent;" in
>>> virtio_vdpa_probe() do the work?
>>>
>>> Thanks,
>>> BR
>>> Zhu Lingshan
>>
>> Good catch.
>>
>> I think we can.
> IMHO you need to specify some 'dma_device', not try and play tricks
> with dma_ops, or assuming the parent is always the device used for
> dma.
>
> Jason

Right, this is what in my mind and discussed in the vhost-vdpa thread.

Will go this way.

Thanks