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

From: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: devicetree@vger.kernel.org, Frank Rowand <frowand.list@gmail.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	iommu@lists.linux-foundation.org,
	Rob Herring <robh+dt@kernel.org>,
	linux-rpi-kernel@lists.infradead.org, will@kernel.org,
	hch@lst.de, linux-arm-kernel@lists.infradead.org,
	robin.murphy@arm.com
Subject: Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711
Date: Thu, 08 Oct 2020 12:05:25 +0200	[thread overview]
Message-ID: <12f33d487eabd626db4c07ded5a1447795eed355.camel@suse.de> (raw)
In-Reply-To: <20201002115541.GC7034@gaia>

[-- Attachment #1.1: Type: text/plain, Size: 2760 bytes --]

Hi Catalin, sorry for the late reply.

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.

Regards,
Nicolas

[-- Attachment #1.2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

[-- Attachment #2: Type: text/plain, Size: 156 bytes --]

_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu