Re: [PATCH V3 2/3] PCI: rcar: Do not abort on too many inbound dma-ranges

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Marek,

On Thu, Oct 17, 2019 at 12:33 AM Marek Vasut <marek.vasut@gmail.com> wrote:
> On 10/17/19 12:26 AM, Rob Herring wrote:
> [...]
> >>>> You can have multiple non-continuous DRAM banks for example. And an
> >>>> entry for SRAM optionally. Each DRAM bank and/or the SRAM should have a
> >>>> separate dma-ranges entry, right ?
> >>>
> >>> Not necessarily. We really only want to define the minimum we have to.
> >>> The ideal system is no dma-ranges. Is each bank at a different
> >>> relative position compared to the CPU's view of the system. That would
> >>> seem doubtful for just DRAM banks. Perhaps DRAM and SRAM could change.
> >>
> >> Is that a question ? Anyway, yes, there is a bit of DRAM below the 32bit
> >> boundary and some more above the 32bit boundary. These two banks don't
> >> need to be continuous. And then you could add the SRAM into the mix.
> >
> > Continuous is irrelevant. My question was in more specific terms is
> > (bank1 addr - bank0 addr) different for CPU's view (i.e phys addr) vs.
> > PCI host view (i.e. bus addr)? If not, then that is 1 translation and
> > 1 dma-ranges entry.
>
> I don't think it's different in that aspect. Except the bus has this
> 32bit limitation, where it only sees subset of the DRAM.
>
> Why should the DMA ranges incorrectly cover also the DRAM which is not
> present ?
>
> >>> I suppose if your intent is to use inbound windows as a poor man's
> >>> IOMMU to prevent accesses to the holes, then yes you would list them
> >>> out. But I think that's wrong and difficult to maintain. You'd also
> >>> need to deal with reserved-memory regions too.
> >>
> >> What's the problem with that? The bootloader has all that information
> >> and can patch the DT correctly. In fact, in my specific case, I have
> >> platform which can be populated with differently sized DRAM, so the
> >> holes are also dynamically calculated ; there is no one DT then, the
> >> bootloader is responsible to generate the dma-ranges accordingly.
> >
> > The problems are it doesn't work:
> >
> > Your dma-mask and offset are not going to be correct.
> >
> > You are running out of inbound windows. Your patch does nothing to
> > solve that. The solution would be merging multiple dma-ranges entries
> > to a single inbound window. We'd have to do that both for dma-mask and
> > inbound windows. The former would also have to figure out which
> > entries apply to setting up dma-mask. I'm simply suggesting just do
> > that up front and avoid any pointless splits.
>
> But then the PCI device can trigger a transaction to non-existent DRAM
> and cause undefined behavior. Surely we do not want that ?

The PCI device will trigger transactions to memory only when instructed
to do so by Linux, right?  Hence if Linux takes into account chosen/memory
and dma-ranges, there is no problem?

> > You are setting up random inbound windows. The bootloader can't assume
> > what order the OS parses dma-ranges, and the OS can't assume what
> > order the bootloader writes the entries.
>
> But the OS can assume the ranges are correct and cover only valid
> memory, right ? That is, memory into which the PCI controller can safely
> access.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds