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

[Nicolas Saenz Julienne]

Hi Jeremy.

On Thu, 2020-10-08 at 22:59 -0500, Jeremy Linton wrote:
> On 10/8/20 2:43 PM, Ard Biesheuvel wrote:
> > (+ 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.
> > It also requires delaying setting the arm64_dma_phy_limit a bit, but 
> that doesn't appear to be causing a problem. I've been boot/compiling 
> with a patch like:
> 
> diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
> index cada0b816c8a..9dfe776c1c75 100644
> --- a/arch/arm64/kernel/acpi.c
> +++ b/arch/arm64/kernel/acpi.c
> @@ -14,6 +14,7 @@
> 
>   #include <linux/acpi.h>
>   #include <linux/cpumask.h>
> +#include <linux/dma-direct.h>
>   #include <linux/efi.h>
>   #include <linux/efi-bgrt.h>
>   #include <linux/init.h>
> @@ -168,6 +169,11 @@ static int __init acpi_fadt_sanity_check(void)
>                  ret = -EINVAL;
>          }
> 
> +       if (!strncmp(table->oem_id, "RPIFDN", ACPI_OEM_ID_SIZE) &&
> +           !strncmp(table->oem_table_id,  "RPI4    ", 
> ACPI_OEM_TABLE_ID_SIZE) &&
> +           table->oem_revision <= 0x200)  {
> +               zone_dma_bits = 30;
> +       }
>   out:
>          /*
>           * acpi_get_table() creates FADT table mapping that
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index cd5caca8a929..6c8aaf1570ce 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -195,6 +195,7 @@ static void __init zone_sizes_init(unsigned long 
> min, unsigned long max)
>          unsigned long max_zone_pfns[MAX_NR_ZONES]  = {0};
> 
>   #ifdef CONFIG_ZONE_DMA
> +       arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
>          max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);
>   #endif
>   #ifdef CONFIG_ZONE_DMA32
> @@ -393,7 +394,6 @@ void __init arm64_memblock_init(void)
>                   */
>                  if (zone_dma_bits == ZONE_DMA_BITS_DEFAULT)
>                          zone_dma_bits = 32;
> -               arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
>          }
> 
>          if (IS_ENABLED(CONFIG_ZONE_DMA32))
> 
> 

Thanks for taking the time to look at this!

Regards,
Nicolas

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