Re: [RFC 2/4] arm64: mm: parse dma-ranges in order to better estimate arm64_dma_phys_limit

From: Catalin Marinas <catalin.marinas@arm.com>
To: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, will@kernel.org,
	robin.murphy@arm.com, m.szyprowski@samsung.com, hch@lst.de,
	phil@raspberrypi.org, stefan.wahren@i2se.com,
	f.fainelli@gmail.com, mbrugger@suse.com,
	Jisheng.Zhang@synaptics.com
Subject: Re: [RFC 2/4] arm64: mm: parse dma-ranges in order to better estimate arm64_dma_phys_limit
Date: Wed, 24 Jul 2019 14:54:25 +0100	[thread overview]
Message-ID: <20190724135425.GB44864@arrakis.emea.arm.com> (raw)
In-Reply-To: <20190717153135.15507-3-nsaenzjulienne@suse.de>

On Wed, Jul 17, 2019 at 05:31:33PM +0200, Nicolas Saenz Julienne wrote:
> The dma physical limit has so far been calculated based on the memory
> size and the assumption that dma would be at least able to address the
> first 4 GB. This turned out no to be true with the Raspberry Pi 4
> which, on it's main interconnect, can only address the first GB of
> memory, even though it might have up to 4 GB.
> 
> With the current miscalculated dma physical limit the contiguous memory
> reserve is located in an inaccessible area for most of the board's
> devices.
> 
> To solve this we now scan the device tree for the 'dma-ranges' property
> on the root or interconnect nodes, which allows us to calculate the
> lowest common denominator dma physical limit. If no dma-ranges is
> available, we'll default to the old scheme.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
> ---
>  arch/arm64/mm/init.c | 61 +++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 57 insertions(+), 4 deletions(-)

I'd rather have this parsing in the core code, returning setting the
minimum DMA mask (or range, address etc.) that covers all devices/buses
described.

-- 
Catalin