Re: [PATCH v3 7/8] virtio/s390: use DMA memory for ccw I/O and classic notifiers

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 4 Jun 2019 15:08:19 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Mon, 3 Jun 2019 18:17:16 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Wed, 29 May 2019 14:26:56 +0200
> > Michael Mueller <mimu@linux.ibm.com> wrote:

> > (...)
> >   
> > > @@ -176,6 +180,22 @@ static struct virtio_ccw_device *to_vc_device(struct virtio_device *vdev)
> > >  	return container_of(vdev, struct virtio_ccw_device, vdev);
> > >  }
> > >  
> > > +static inline void *__vc_dma_alloc(struct virtio_device *vdev, size_t size)
> > > +{
> > > +	return ccw_device_dma_zalloc(to_vc_device(vdev)->cdev, size);
> > > +}
> > > +
> > > +static inline void __vc_dma_free(struct virtio_device *vdev, size_t size,
> > > +				 void *cpu_addr)
> > > +{
> > > +	return ccw_device_dma_free(to_vc_device(vdev)->cdev, cpu_addr, size);
> > > +}
> > > +
> > > +#define vc_dma_alloc_struct(vdev, ptr) \
> > > +	({ptr = __vc_dma_alloc(vdev, sizeof(*(ptr))); })
> > > +#define vc_dma_free_struct(vdev, ptr) \
> > > +	__vc_dma_free(vdev, sizeof(*(ptr)), (ptr))
> > > +  
> > 
> > I *still* don't like these #defines (and the __vc_dma_* functions), as I
> > already commented last time. I think they make it harder to follow the
> > code.
> >   
> 
> Sorry! I think we simply forgot to address this comment of yours. 
> 
> > >  static void drop_airq_indicator(struct virtqueue *vq, struct airq_info *info)
> > >  {
> > >  	unsigned long i, flags;
> > > @@ -336,8 +356,7 @@ static void virtio_ccw_drop_indicator(struct virtio_ccw_device *vcdev,
> > >  	struct airq_info *airq_info = vcdev->airq_info;
> > >  
> > >  	if (vcdev->is_thinint) {
> > > -		thinint_area = kzalloc(sizeof(*thinint_area),
> > > -				       GFP_DMA | GFP_KERNEL);
> > > +		vc_dma_alloc_struct(&vcdev->vdev, thinint_area);  
> > 
> > Last time I wrote:
> > 
> > "Any reason why this takes a detour via the virtio device? The ccw
> >  device is already referenced in vcdev, isn't it?
> >
> > thinint_area = ccw_device_dma_zalloc(vcdev->cdev, sizeof(*thinint_area));
> > 
> >  looks much more obvious to me."
> > 
> > It still seems more obvious to me.
> >  
> 
> 
> The reason why I decided to introduce __vc_dma_alloc() back then is
> because I had no clarity what do we want to do there. If you take a look
> the body of __vc_dma_alloc() changed quite a lot, while I the usage not
> so much. 
> 
> Regarding why is the first argument a pointer struct virtio_device, the
> idea was probably to keep the needs to be ZONE_DMA and can use the full
> 64 bit address space separate. But I abandoned the ideal.
> 
> Also vc_dma_alloc_struct() started out more elaborate (I used to manage
> a dma_addr_t as well -- see RFC).

Understood, history is often important :)

> 
> I'm not quite sure what is your problem with the these. As far as I
> understand, this is another of those matter of taste things. But it ain't
> a big deal. 

Two things:
- The call path goes from the vcdev to the vdev, then back to the vcdev
  and then to the cdev. Going from the vcdev to the cdev  directly
  eliminates the roundtrip via the vdev, which I think does not add
  anything.
- I prefer
	variable = function_returning_a_pointer(...);
  over
	function_setting_a_variable(..., variable);
  The latter obscures the fact that we change the value of the
  variable, unless named very obviously.

> 
> I will change this for v4 as you requested. Again sorry for missing it!

np, can happen.

> 
> Regards,
> Halil
> 
>  
> > >  		if (!thinint_area)
> > >  			return;
> > >  		thinint_area->summary_indicator =  
> >   
>