linux-kernel - Re: [PATCH 12/16] staging/comedi: mark as broken

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Message-ID: <60c6af3d-d8e4-5745-8d2b-9791a2f4ff56@mev.co.uk>
Date:   Mon, 17 Jun 2019 14:15:43 +0100
From:   Ian Abbott <abbotti@....co.uk>
To:     Christoph Hellwig <hch@....de>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        H Hartley Sweeten <hsweeten@...ionengravers.com>,
        devel@...verdev.osuosl.org, linux-s390@...r.kernel.org,
        Intel Linux Wireless <linuxwifi@...el.com>,
        linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, linux-wireless@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH 12/16] staging/comedi: mark as broken

On 14/06/2019 16:34, Christoph Hellwig wrote:
> On Fri, Jun 14, 2019 at 05:30:32PM +0200, Greg KH wrote:
>> On Fri, Jun 14, 2019 at 04:48:57PM +0200, Christoph Hellwig wrote:
>>> On Fri, Jun 14, 2019 at 04:02:39PM +0200, Greg KH wrote:
>>>> Perhaps a hint as to how we can fix this up?  This is the first time
>>>> I've heard of the comedi code not handling dma properly.
>>>
>>> It can be fixed by:
>>>
>>>   a) never calling virt_to_page (or vmalloc_to_page for that matter)
>>>      on dma allocation
>>>   b) never remapping dma allocation with conflicting cache modes
>>>      (no remapping should be doable after a) anyway).
>>
>> Ok, fair enough, have any pointers of drivers/core code that does this
>> correctly?  I can put it on my todo list, but might take a week or so...
> 
> Just about everyone else.  They just need to remove the vmap and
> either do one large allocation, or live with the fact that they need
> helpers to access multiple array elements instead of one net vmap,
> which most of the users already seem to do anyway, with just a few
> using the vmap (which might explain why we didn't see blowups yet).

Avoiding the vmap in comedi should be do-able as it already has other 
means to get at the buffer pages.

When comedi makes the buffer from DMA coherent memory, it currently 
allocates it as a series of page-sized chunks.  That cannot be mmap'ed 
in one go with dma_mmap_coherent(), so I see the following solutions.

1. Change the buffer allocation to allocate a single chunk of DMA 
coherent memory and use dma_mmap_coherent() to mmap it.

2. Call dma_mmap_coherent() in a loop, adjusting vma->vm_start and 
vma->vm_end for each iteration (vma->vm_pgoff will be 0), and restoring 
the vma->vm_start and vma->vm_end at the end.

I'm not sure if 2 is a legal option.

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