Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711

[Ard Biesheuvel]

(+ Lorenzo)

On Thu, 8 Oct 2020 at 12:14, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Thu, Oct 08, 2020 at 12:05:25PM +0200, Nicolas Saenz Julienne wrote:
> > On Fri, 2020-10-02 at 12:55 +0100, Catalin Marinas wrote:
> > > On Thu, Oct 01, 2020 at 07:31:19PM +0200, Nicolas Saenz Julienne wrote:
> > > > On Thu, 2020-10-01 at 18:23 +0100, Catalin Marinas wrote:
> > > > > On Thu, Oct 01, 2020 at 06:15:01PM +0100, Catalin Marinas wrote:
> > > > > > On Thu, Oct 01, 2020 at 06:17:37PM +0200, Nicolas Saenz Julienne wrote:
> > > > > > > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> > > > > > > index 4602e467ca8b..cd0d115ef329 100644
> > > > > > > --- a/drivers/of/fdt.c
> > > > > > > +++ b/drivers/of/fdt.c
> > > > > > > @@ -25,6 +25,7 @@
> > > > > > >  #include <linux/serial_core.h>
> > > > > > >  #include <linux/sysfs.h>
> > > > > > >  #include <linux/random.h>
> > > > > > > +#include <linux/dma-direct.h>      /* for zone_dma_bits */
> > > > > > >
> > > > > > >  #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
> > > > > > >  #include <asm/page.h>
> > > > > > > @@ -1198,6 +1199,14 @@ void __init early_init_dt_scan_nodes(void)
> > > > > > >     of_scan_flat_dt(early_init_dt_scan_memory, NULL);
> > > > > > >  }
> > > > > > >
> > > > > > > +void __init early_init_dt_update_zone_dma_bits(void)
> > > > > > > +{
> > > > > > > +   unsigned long dt_root = of_get_flat_dt_root();
> > > > > > > +
> > > > > > > +   if (of_flat_dt_is_compatible(dt_root, "brcm,bcm2711"))
> > > > > > > +           zone_dma_bits = 30;
> > > > > > > +}
> > > > > >
> > > > > > I think we could keep this entirely in the arm64 setup_machine_fdt() and
> > > > > > not pollute the core code with RPi4-specific code.
> > > > >
> > > > > Actually, even better, could we not move the check to
> > > > > arm64_memblock_init() when we initialise zone_dma_bits?
> > > >
> > > > I did it this way as I vaguely remembered Rob saying he wanted to centralise
> > > > all early boot fdt code in one place. But I'll be happy to move it there.
> > >
> > > I can see Rob replied and I'm fine if that's his preference. However,
> > > what I don't particularly like is that in the arm64 code, if
> > > zone_dma_bits == 24, we set it to 32 assuming that it wasn't touched by
> > > the early_init_dt_update_zone_dma_bits(). What if at some point we'll
> > > get a platform that actually needs 24 here (I truly hope not, but just
> > > the principle of relying on magic values)?
> > >
> > > So rather than guessing, I'd prefer if the arch code can override
> > > ZONE_DMA_BITS_DEFAULT. Then, in arm64, we'll just set it to 32 and no
> > > need to explicitly touch the zone_dma_bits variable.
> >
> > Yes, sonds like the way to go. TBH I wasn't happy with that solution either,
> > but couldn't think of a nicer alternative.
> >
> > Sadly I just realised that the series is incomplete, we have RPi4 users that
> > want to boot unsing ACPI, and this series would break things for them. I'll
> > have a word with them to see what we can do for their use-case.
>
> Is there a way to get some SoC information from ACPI?
>

This is unfortunate. We used ACPI _DMA methods as they were designed
to communicate the DMA limit of the XHCI controller to the OS.

It shouldn't be too hard to match the OEM id field in the DSDT, and
switch to the smaller mask. But it sucks to have to add a quirk like
that.