linux-kernel - Re: [RFC 3/4] dma-direct: add dma_direct_min

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Message-ID: <20190718091526.GA25321@lst.de>
Date:   Thu, 18 Jul 2019 11:15:26 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, catalin.marinas@....com,
        will@...nel.org, phil@...pberrypi.org, stefan.wahren@...e.com,
        f.fainelli@...il.com, mbrugger@...e.com,
        Jisheng.Zhang@...aptics.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/4] dma-direct: add dma_direct_min_mask

On Wed, Jul 17, 2019 at 05:31:34PM +0200, Nicolas Saenz Julienne wrote:
> Historically devices with ZONE_DMA32 have been assumed to be able to
> address at least the lower 4GB of ram for DMA. This is still the defualt
> behavior yet the Raspberry Pi 4 is limited to the first GB of memory.
> This has been observed to trigger failures in dma_direct_supported() as
> the 'min_mask' isn't properly set.
> 
> We create 'dma_direct_min_mask' in order for the arch init code to be
> able to fine-tune dma direct's 'min_dma' mask.

Normally we use ZONE_DMA for that case.