linux-kernel - Re: [PATCH RFC v2 2/5] of: get dma area lower limit

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Message-ID: <ZnH-VU2iz9Q2KLbr@arm.com>
Date: Tue, 18 Jun 2024 22:38:29 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Baruch Siach <baruch@...s.co.il>
Cc: Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Rob Herring <robh@...nel.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	iommu@...ts.linux.dev, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	Petr Tesařík <petr@...arici.cz>,
	Ramon Fried <ramon@...reality.ai>,
	Elad Nachman <enachman@...vell.com>
Subject: Re: [PATCH RFC v2 2/5] of: get dma area lower limit

On Tue, Apr 09, 2024 at 09:17:55AM +0300, Baruch Siach wrote:
> of_dma_get_max_cpu_address() returns the highest CPU address that
> devices can use for DMA. The implicit assumption is that all CPU
> addresses below that limit are suitable for DMA. However the
> 'dma-ranges' property this code uses also encodes a lower limit for DMA
> that is potentially non zero.
> 
> Rename to of_dma_get_cpu_limits(), and extend to retrieve also the lower
> limit for the same 'dma-ranges' property describing the high limit.

I don't understand the reason for the lower limit. The way the Linux
zones work is that ZONE_DMA always starts from the start of the RAM. It
doesn't matter whether it's 0 or not, you'd not allocate below the start
of RAM anyway. If you have a device that cannot use the bottom of the
RAM, it is pretty broken and not supported by Linux.

I think you added this limit before we tried to move away from
zone_dma_bits to a non-power-of-two limit (zone_dma_limit). With the
latter, we no longer need tricks with the lower limit,
of_dma_get_max_cpu_address() should capture the smallest upper CPU
address limit supported by all devices (and that's where ZONE_DMA should
end).

-- 
Catalin