linux-kernel - Re: [RFC v2 2/2] base: dma-mapping: Postpone page_to

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Date:   Mon, 09 Apr 2018 18:18:23 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Rich Felker <dalias@...c.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        jacopo mondi <jacopo@...ndi.org>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        ysato@...rs.sourceforge.jp, geert@...ux-m68k.org,
        linux-renesas-soc@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org
Subject: Re: [RFC v2 2/2] base: dma-mapping: Postpone page_to_pfn() on mmap()

Hi Rich,

On Monday, 9 April 2018 18:11:13 EEST Rich Felker wrote:
> On Mon, Apr 09, 2018 at 04:06:15PM +0300, Laurent Pinchart wrote:
> > On Monday, 9 April 2018 14:11:22 EEST Robin Murphy wrote:
> >> On 09/04/18 08:25, jacopo mondi wrote:
> >>> Hi Robin, Laurent,
> >>> 
> >>>      a long time passed, sorry about this.
> >>> 
> >>> On Wed, Nov 15, 2017 at 01:38:23PM +0000, Robin Murphy wrote:
> >>>> On 14/11/17 17:08, Jacopo Mondi wrote:
> >>>>> On SH4 architecture, with SPARSEMEM memory model, translating page
> >>>>> to pfn hangs the CPU. Post-pone translation to pfn after
> >>>>> dma_mmap_from_dev_coherent() function call as it succeeds and make
> >>>>> page translation not necessary.
> >>>>> 
> >>>>> This patch was suggested by Laurent Pinchart and he's working to
> >>>>> submit a proper fix mainline. Not sending for inclusion at the
> >>>>> moment.
> >>>> 
> >>>> Y'know, I think this patch does have some merit by itself - until we
> >>>> know that cpu_addr *doesn't* represent some device-private memory
> >>>> which is not guaranteed to be backed by a struct page, calling
> >>>> virt_to_page() on it is arguably semantically incorrect, even if it
> >>>> might happen to be benign in most cases.
> >>> 
> >>> I still need to carry this patch in my trees to have a working dma
> >>> memory mapping on SH4 platforms. My understanding from your comment is
> >>> that there may be a way forward for this patch, do you still think the
> >>> same? Have you got any suggestion on how to improve this eventually
> >>> for inclusion?
> >> 
> >> As before, the change itself does seem reasonable; it might be worth
> >> rewording the commit message in more general terms rather than making it
> >> sound like an SH-specific workaround (which I really don't think it is),
> >> but otherwise I'd say just repost it as a non-RFC patch.
> > 
> > I actually can't remember any better fix I would have in mind, so this
> > looks good to me :-) I agree with Robin, the commit message should be
> > reworded. Robin's explanation of why virt_to_page() should be postponed
> > until we know that cpu_addr represents memory that is guaranteed to be
> > backed by a struct page is a good starting point. You can mention SH4 as
> > an example of an architecture that will crash when calling virt_to_page()
> > in such a case, but the fix isn't specific to SH4.
> 
> Just checking since I joined late -- is the consensus that this does
> not require any action/review specific to SH (in my role as maintainer
> way behind on maintenance)?

I don't think it requires any action specific to SH. If you want to review the 
patch in the context of SH though, please feel free to do so :-)

-- 
Regards,

Laurent Pinchart