Re: [Xen-devel] [PATCH 00/11] of: Fix DMA configuration for non-DT masters

[Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>]

On Wed, 2019-09-25 at 16:09 +0100, Robin Murphy wrote:
> On 25/09/2019 15:52, Nicolas Saenz Julienne wrote:
> > On Tue, 2019-09-24 at 16:59 -0500, Rob Herring wrote:
> > > On Tue, Sep 24, 2019 at 1:12 PM Nicolas Saenz Julienne
> > > <nsaenzjulienne@xxxxxxx> wrote:
> > > > Hi All,
> > > > this series tries to address one of the issues blocking us from
> > > > upstreaming Broadcom's STB PCIe controller[1]. Namely, the fact that
> > > > devices not represented in DT which sit behind a PCI bus fail to get the
> > > > bus' DMA addressing constraints.
> > > > 
> > > > This is due to the fact that of_dma_configure() assumes it's receiving a
> > > > DT node representing the device being configured, as opposed to the PCIe
> > > > bridge node we currently pass. This causes the code to directly jump
> > > > into PCI's parent node when checking for 'dma-ranges' and misses
> > > > whatever was set there.
> > > > 
> > > > To address this I create a new API in OF - inspired from Robin Murphys
> > > > original proposal[2] - which accepts a bus DT node as it's input in
> > > > order to configure a device's DMA constraints. The changes go deep into
> > > > of/address.c's implementation, as a device being having a DT node
> > > > assumption was pretty strong.
> > > > 
> > > > On top of this work, I also cleaned up of_dma_configure() removing its
> > > > redundant arguments and creating an alternative function for the special
> > > > cases
> > > > not applicable to either the above case or the default usage.
> > > > 
> > > > IMO the resulting functions are more explicit. They will probably
> > > > surface some hacky usages that can be properly fixed as I show with the
> > > > DT fixes on the Layerscape platform.
> > > > 
> > > > This was also tested on a Raspberry Pi 4 with a custom PCIe driver and
> > > > on a Seattle AMD board.
> > > 
> > > Humm, I've been working on this issue too. Looks similar though yours
> > > has a lot more churn and there's some other bugs I've found.
> > 
> > That's good news, and yes now that I see it, some stuff on my series is
> > overly
> > complicated. Specially around of_translate_*().
> > 
> > On top of that, you removed in of_dma_get_range():
> > 
> > -   /*
> > -    * At least empty ranges has to be defined for parent node if
> > -    * DMA is supported
> > -    */
> > -   if (!ranges)
> > -           break;
> > 
> > Which I assumed was bound to the standard and makes things easier.
> > 
> > > Can you test out this branch[1]. I don't have any h/w needing this,
> > > but wrote a unittest and tested with modified QEMU.
> > 
> > I reviewed everything, I did find a minor issue, see the patch attached.
> 
> WRT that patch, the original intent of "force_dma" was purely to 
> consider a device DMA-capable regardless of the presence of 
> "dma-ranges". Expecting of_dma_configure() to do anything for a non-OF 
> device has always been bogus - magic paravirt devices which appear out 
> of nowhere and expect to be treated as genuine DMA masters are a 
> separate problem that we haven't really approached yet.

I agree it's clearly abusing the function. I have no problem with the behaviour
change if it's OK with you.

Robin, have you looked into supporting multiple dma-ranges? It's the next thing
we need for BCM STB's PCIe. I'll have a go at it myself if nothing is in the
works already.

Regards,
Nicolas

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