linux-kernel - Re: [PATCH 8/8] WIP: add a dma_alloc

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Message-ID: <20201110095747.GA26574@lst.de>
Date:   Tue, 10 Nov 2020 10:57:47 +0100
From:   Christoph Hellwig <hch@....de>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Ricardo Ribalda <ribalda@...omium.org>,
        Christoph Hellwig <hch@....de>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [PATCH 8/8] WIP: add a dma_alloc_contiguous API

On Tue, Nov 10, 2020 at 06:50:32PM +0900, Tomasz Figa wrote:
> In what terms it doesn't actually work? Last time I checked some
> platforms actually defined CONFIG_DMA_NONCOHERENT, so those would
> instead use the kmalloc() + dma_map() path. I don't have any
> background on why that was added and whether it needs to be preserved,
> though. Kieran, Laurent, do you have any insight?

CONFIG_DMA_NONCOHERENT is set on sh and mips for platforms that may
support non-coherent DMA at compile time (but at least for mips that
doesn't actually means this gets used).  Using that ifdef to decide
on using usb_alloc_coherent vs letting the usb layer map the data
seems at best odd, and if we are unlucky papering over a bug somewhere.