linux-kernel - Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve per-numa CMA

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Message-ID: <20200722141658.GA17658@lst.de>
Date:   Wed, 22 Jul 2020 16:16:58 +0200
From:   Christoph Hellwig <hch@....de>
To:     Barry Song <song.bao.hua@...ilicon.com>
Cc:     hch@....de, m.szyprowski@...sung.com, robin.murphy@....com,
        will@...nel.org, ganapatrao.kulkarni@...ium.com,
        catalin.marinas@....com, iommu@...ts.linux-foundation.org,
        linuxarm@...wei.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Steve Capper <steve.capper@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve
 per-numa CMA

On Sun, Jun 28, 2020 at 11:12:50PM +1200, Barry Song wrote:
> This is useful for at least two scenarios:
> 1. ARM64 smmu will get memory from local numa node, it can save its
> command queues and page tables locally. Tests show it can decrease
> dma_unmap latency at lot. For example, without this patch, smmu on
> node2 will get memory from node0 by calling dma_alloc_coherent(),
> typically, it has to wait for more than 560ns for the completion of
> CMD_SYNC in an empty command queue; with this patch, it needs 240ns
> only.
> 2. when we set iommu passthrough, drivers will get memory from CMA,
> local memory means much less latency.

I really don't like the config options.  With the boot parameters
you can always hardcode that in CONFIG_CMDLINE anyway.