[PATCH v5 6/9] ARM: shmobile: Add PCIe device tree nodes for R8A7790

[Phil.Edworthy at renesas.com]

Hi Arnd,

On: 26/03/2014 11:53, Arnd wrote:
> Subject: Re: [PATCH v5 6/9] ARM: shmobile: Add PCIe device tree nodes 
for R8A7790
> 
> On Wednesday 26 March 2014 11:34:41 Phil.Edworthy at renesas.com wrote:
> > > But the ranges you specified in the property don't actually fit in 
those
> > > constraints: you have a range with size 0x8000000 and start 
0x40000000,
> > > which you say can't be programmed into the hardware.
> > 
> > Actually, the driver checks the dma-ranges against these constraints, 
and 
> > if necessary will create multiple mappings to fulfil the requested 
> > dma-ranges.
> 
> Ok, I didn't notice. My initial suggestion was to not put that logic
> into the driver but instead specify in the host bridge binding that each
> entry in the dma-ranges property has to meet the hardware constraints.
> 
> As long as you don't have too much complexity to detect this case, I'm
> fine with it either way.
Ok, good!

> > > > Still, my comment about the OF PCI range code treating both 32 and 

> > 64-bit 
> > > > types the same way means that PCIe host driver has to assume it's 
a 
> > 64-bit 
> > > > mapping.
> > > 
> > > I was thinking more of PCI devices than the host itself. If the host
> > > driver can verify that all mappings are in the first 4GB and cover 
all 
> > of
> > > RAM, we won't have to use an swiotlb for devices that don't support 
> > 64-bit
> > > DMA, which is a very significant performance difference.
> > Ok, I think I understand. However, all the other PCI host drivers just 
do 
> > 1-to-1 mapping between PCI and CPU addresses, right? Whilst it might 
be 
> > nice be able to support mapping CPU addresses > 4GiB to PCI addresses 
> > under 4GiB, can that be something to consider later on?
> 
> Yes, fair enough. The current version is much simpler, so that's ok.
> Just keep it in mind if you run into performance problems. Also, note
> that we don't actually support swiotlb on arm32 yet, so your current
> code is broken for an PCI DMA master that is not 64-bit capable.
> We need swiotlb on arm32 anyway, and that will fix this problem, but
> adding the hack I described would also fix it.
Ok, thanks for all your comments!

Phil