Re: [Xen-devel] [PATCH v7 3/8] arm64: introduce is_device_dma_coherent

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 4 Nov 2014, Grygorii Strashko wrote:
> Hi Stefano,
> 
> On 11/03/2014 01:10 PM, Stefano Stabellini wrote:
> > On Mon, 3 Nov 2014, Will Deacon wrote:
> >> On Mon, Nov 03, 2014 at 10:46:03AM +0000, Stefano Stabellini wrote:
> >>> On Mon, 27 Oct 2014, Stefano Stabellini wrote:
> >>>> Introduce a boolean flag and an accessor function to check whether a
> >>>> device is dma_coherent. Set the flag from set_arch_dma_coherent_ops.
> >>>>
> >>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> >>>> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> >>>> CC: will.deacon@xxxxxxx
> >>>
> >>> Will, Catalin,
> >>> are you OK with this patch?
> >>
> >> It would be nicer if the dma_coherent flag didn't have to be duplicated by
> >> each architecture in dev_archdata. Is there any reason not to put it in the
> >> core code?
> > 
> > Yes, there is a reason for it: if I added a boolean dma_coherent flag in
> > struct device as Catalin initially suggested, what would be the default
> > for each architecture? Where would I set it for arch that don't use
> > device tree? It is not easy.
> > 
> > I thought it would be better to introduce is_device_dma_coherent only on
> > the architectures where it certainly makes sense to have it. In fact I
> > checked and arm and arm64 are the only architectures to define
> > set_arch_dma_coherent_ops at the moment. At that point if
> > is_device_dma_coherent becomes arch-specific, it makes sense to store
> > the flag in dev_archdata instead of struct device.
> 
> The proposition from Will looks reasonable for me too, because
> there is "small" side-effect of adding such kind of properties to
> arch-specific data or even to the core device structure. ;(
> 
> There are some sub-systems in kernel which do not create their devices
> from DT and instead some host device populates its children devices manually.
>  Now, I know at least two cases:
> - usb: dwc3 core creates xhci device manually
> - pci: adds its client devices
> 
> In such, case DMA configuration have to be propagated from host to
> child (in our case host device's got DMA configuration from DT), like:
>       dma_set_coherent_mask(&xhci->dev, dwc->dev->coherent_dma_mask);
> 
>       xhci->dev.parent        = dwc->dev;
>       xhci->dev.dma_mask      = dwc->dev->dma_mask;
>       xhci->dev.dma_parms     = dwc->dev->dma_parms;
> 
> So, once new DMA property is added it has to be propagated from 
> host to child device too.
> 
> Recently, the new property  dma_pfn_offset was introduced in struct device 
> and such kind of problem was observed on keystone 2:
> - for usb case it was fixed using Platform Bus notifier (xhci - platform 
> device)
> - for pci - the work is in progress, because solution with PCI Bus notifier
>   was rejected https://lkml.org/lkml/2014/10/10/308.
> 
> In general, if dma_coherent will belong to struct device then
> such problems will be possible to fix directly in drivers/subsystems:
> xhci->dev.dma_coherent        = dwc->dev->dma_coherent;
> 
> But, if it will be arch-specific data then it will be impossible to
> set it without introducing proper and arch-specific setters/getters functions.
>
> Also, as an idea, we are thinking about introducing something like:
>   void dma_apply_parent_cfg(struct device *dev, struct device *parent)
> which will ensure that all DMA configuration properly copied from
> parent to children device. Now it should be (as minimum for ARM):
>  dma_mask
>  coherent_dma_mask
>  dma_parms
>  dma_pfn_offset
>  dev_archdata->dma_ops
>  [dma_coherent]?

I understand your concern but the problem you have goes far beyond a
simple dma_coherent flag: what about all the other dev_archdata fields?
Aside from dma_ops, on some other architectures there might be other
data structrures in dev_archdata that need to be properly initialized
from the parent.

Your idea of introducing something like dma_apply_parent_cfg is the only
solid solution I can see.  However I would consider naming it something
more generic like init_dev_from_parent to handle other possible
configurations (inside or outside dev_archdata) that might have to be
initialized from information on the parent device.


Regarding the dma_coherent flag, I still prefer this approach because
introducing the dma_coherent flag in dev_archdata wouldn't make this
issue any worse than already is, but would avoid other problems as
mentioned in my previous reply.

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