linux-kernel - Re: [PATCH v3] Add udmabuf misc device

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Date:   Tue, 29 May 2018 14:30:55 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Shuah Khan <shuah@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v3] Add udmabuf misc device

On Tue, May 29, 2018 at 10:48 AM, Gerd Hoffmann <kraxel@...hat.com> wrote:
>   Hi,
>
>> > +static void *kmap_atomic_udmabuf(struct dma_buf *buf, unsigned long page_num)
>> > +{
>> > +   struct udmabuf *ubuf = buf->priv;
>> > +   struct page *page = ubuf->pages[page_num];
>> > +
>> > +   return kmap_atomic(page);
>> > +}
>> > +
>> > +static void *kmap_udmabuf(struct dma_buf *buf, unsigned long page_num)
>> > +{
>> > +   struct udmabuf *ubuf = buf->priv;
>> > +   struct page *page = ubuf->pages[page_num];
>> > +
>> > +   return kmap(page);
>> > +}
>>
>> The above leaks like mad since no kunamp?
>
> /me checks code.  Oops.  Yes.
>
> The docs say map() is required and unmap() is not (for both atomic and
> non-atomic cases), so I assumed there is a default implementation just
> doing kunmap(page).  Which is not the case.  /me looks a bit surprised.
>
> I'll fix it for v4.
>
>> Also I think we have 0 users of the kmap atomic interfaces ... so not sure
>> whether it's worth it to implement those.
>
> Well, the docs are correct.  kmap_atomic() is required, dma-buf.c calls
> the function pointer without checking it exists beforehand ...

Frankly with the pletoria of dummy kmap functions that just return
NULL; it might be better to move that into core dma-buf code and make
it optional for real. Since it's indeed very surprising.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch