linux-kernel - Re: [PATCH 08/18] dma-mapping: add a new dma_alloc

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Message-ID: <20200925161754.GA18721@lst.de>
Date:   Fri, 25 Sep 2020 18:17:54 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Matt Porter <mporter@...nel.crashing.org>,
        iommu@...ts.linux-foundation.org, alsa-devel@...a-project.org,
        linux-samsung-soc@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-doc@...r.kernel.org,
        nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-mm@...ck.org,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        netdev@...r.kernel.org, linux1394-devel@...ts.sourceforge.net,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 08/18] dma-mapping: add a new dma_alloc_noncoherent API

On Fri, Sep 25, 2020 at 12:15:37PM +0100, Robin Murphy wrote:
> On 2020-09-15 16:51, Christoph Hellwig wrote:
> [...]
>> +These APIs allow to allocate pages in the kernel direct mapping that are
>> +guaranteed to be DMA addressable.  This means that unlike dma_alloc_coherent,
>> +virt_to_page can be called on the resulting address, and the resulting
>
> Nit: if we explicitly describe this as if it's a guarantee that can be 
> relied upon...
>
>> +struct page can be used for everything a struct page is suitable for.
>
> [...]
>> +This routine allocates a region of <size> bytes of consistent memory.  It
>> +returns a pointer to the allocated region (in the processor's virtual address
>> +space) or NULL if the allocation failed.  The returned memory may or may not
>> +be in the kernels direct mapping.  Drivers must not call virt_to_page on
>> +the returned memory region.
>
> ...then forbid this document's target audience from relying on it, 
> something seems off. At the very least it's unhelpfully unclear :/
>
> Given patch #17, I suspect that the first paragraph is the one that's no 
> longer true.

Yes.  dma_alloc_pages is the replacement for allocations that need the
direct mapping.  I'll send a patch to document dma_alloc_pages and
fixes this up