linux-kernel - Re: [PATCH] dma-mapping: treat dev->bus_dma

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Message-ID: <f74cd8a6-00bf-46c3-8e2e-d278e72d6e0e@arm.com>
Date:   Wed, 13 Nov 2019 20:34:15 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Burton <paulburton@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
        phil@...pberrypi.org, linux-acpi@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        James Hogan <jhogan@...nel.org>, Len Brown <lenb@...nel.org>,
        devicetree@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-mips@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] dma-mapping: treat dev->bus_dma_mask as a DMA limit

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.

[...]
> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
> index 5a7551d060f2..f18827cf96df 100644
> --- a/drivers/acpi/arm64/iort.c
> +++ b/drivers/acpi/arm64/iort.c
> @@ -1097,7 +1097,7 @@ void iort_dma_setup(struct device *dev, u64 *dma_addr, u64 *dma_size)
>   		 * Limit coherent and dma mask based on size
>   		 * retrieved from firmware.
>   		 */
> -		dev->bus_dma_mask = mask;
> +		dev->bus_dma_limit = mask;

Although this preserves the existing behaviour, as in of_dma_configure() 
we can do better here since we have the original address range to hand. 
I think it's worth keeping the ACPI and OF paths in sync for minor 
tweaks like this, rather than letting them diverge unnecessarily.

Otherwise, the rest looks OK to me - in principle we could store it as 
an exclusive limit such that we could then streamline the min_not_zero() 
tests to just min(mask, limit - 1), but that's probably too clever for 
its own good.

Robin.

>   		dev->coherent_dma_mask = mask;
>   		*dev->dma_mask = mask;
>   	}