Re: [PATCH] dma-mapping: treat dev->bus_dma_mask as a DMA limit

From: Florian Fainelli <f.fainelli@gmail.com>
To: Robin Murphy <robin.murphy@arm.com>,
	Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Hanjun Guo <guohanjun@huawei.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	 Jens Axboe <axboe@kernel.dk>, Joerg Roedel <joro@8bytes.org>,
	Rob Herring <robh+dt@kernel.org>,
	Frank Rowand <frowand.list@gmail.com>,
	Christoph Hellwig <hch@lst.de>,
	Marek Szyprowski <m.szyprowski@samsung.com>
Cc: linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-ide@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Paul Burton <paulburton@kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	x86@kernel.org, phil@raspberrypi.org, linux-acpi@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	James Hogan <jhogan@kernel.org>, Len Brown <lenb@kernel.org>,
	devicetree@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	linux-mips@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>,
	iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] dma-mapping: treat dev->bus_dma_mask as a DMA limit
Date: Wed, 13 Nov 2019 12:41:33 -0800	[thread overview]
Message-ID: <48da05e0-5acf-8ab3-a6c9-be4988a9450b@gmail.com> (raw)
In-Reply-To: <f74cd8a6-00bf-46c3-8e2e-d278e72d6e0e@arm.com>

On 11/13/19 12:34 PM, Robin Murphy wrote:
> On 13/11/2019 4:13 pm, Nicolas Saenz Julienne wrote:
>> Using a mask to represent bus DMA constraints has a set of limitations.
>> The biggest one being it can only hold a power of two (minus one). The
>> DMA mapping code is already aware of this and treats dev->bus_dma_mask
>> as a limit. This quirk is already used by some architectures although
>> still rare.
>>
>> With the introduction of the Raspberry Pi 4 we've found a new contender
>> for the use of bus DMA limits, as its PCIe bus can only address the
>> lower 3GB of memory (of a total of 4GB). This is impossible to represent
>> with a mask. To make things worse the device-tree code rounds non power
>> of two bus DMA limits to the next power of two, which is unacceptable in
>> this case.
>>
>> In the light of this, rename dev->bus_dma_mask to dev->bus_dma_limit all
>> over the tree and treat it as such. Note that dev->bus_dma_limit is
>> meant to contain the higher accesible DMA address.
> 
> Neat, you win a "why didn't I do it that way in the first place?" :)
> 
> Looking at it without all the history of previous attempts, this looks
> entirely reasonable, and definitely a step in the right direction.

And while you are changing those, would it make sense to not only rename
the structure member but introduce a getter and setter in order to ease
future work where this would no longer be a scalar?
-- 
Florian
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