From: Baolin Wang <baolin.wang@linaro.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: hch@lst.de, Marek Szyprowski <m.szyprowski@samsung.com>,
Greg KH <gregkh@linuxfoundation.org>,
iommu@lists.linux-foundation.org,
LKML <linux-kernel@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>, Mark Brown <broonie@kernel.org>
Subject: Re: [PATCH 2/2] dma-coherent: Change the bitmap APIs
Date: Sun, 3 Jun 2018 10:11:55 +0800 [thread overview]
Message-ID: <CAMz4kuKM4mKyzh8Ws=82RgtOdLa8d_Dsq-=P0grqpGa=e=q3bQ@mail.gmail.com> (raw)
In-Reply-To: <8ef2eacf-97af-3449-c3ef-c64516ee05ea@arm.com>
On 1 June 2018 at 01:38, Robin Murphy <robin.murphy@arm.com> wrote:
> On 31/05/18 06:55, Baolin Wang wrote:
>>
>> The device coherent memory uses the bitmap helper functions, which take an
>> order of PAGE_SIZE, that means the pages size is always a power of 2 as
>> the
>> allocation region. For Example, allocating 33 MB from a 33 MB dma_mem
>> region
>> requires 64MB free memory in that region.
>>
>> Thus we can change to use bitmap_find_next_zero_area()/bitmap_set()/
>> bitmap_clear() to present the allocation coherent memory, and reduce the
>> allocation granularity to be one PAGE_SIZE.
>>
>> Moreover from Arnd's description:
>> "I believe that bitmap_allocate_region() was chosen here because it is
>> more efficient than bitmap_find_next_zero_area(), at least that is the
>> explanation given in
>> https://en.wikipedia.org/wiki/Buddy_memory_allocation.
>>
>> It's quite possible that we don't actually care about efficiency of
>> dma_alloc_*() since a) it's supposed to be called very rarely, and
>> b) the overhead of accessing uncached memory is likely higher than the
>> search through a relatively small bitmap".
>>
>> Thus I think we can convert to change the allocation granularity to be
>> one PAGE_SIZE replacing with new bitmap APIs, which will not cause
>> efficiency issue.
>
>
> To quote the DMA API docs:
>
> "The CPU virtual address and the DMA address are both
> guaranteed to be aligned to the smallest PAGE_SIZE order which
> is greater than or equal to the requested size. This invariant
> exists (for example) to guarantee that if you allocate a chunk
> which is smaller than or equal to 64 kilobytes, the extent of the
> buffer you receive will not cross a 64K boundary."
>
> Now obviously there's a point above which that stops being practical, but
Agree.
> it's probably safe to assume that a device asking for a multi-megabyte
> buffer doesn't actually have stringent boundary requirements either. For
> smaller sizes, though, it definitely can matter. At the very least up to
> 64KB, and probably up as far as MAX_ORDER-size requests, we need to preserve
> the existing behaviour or something, somewhere, will break.
Some devices really don't care the boundary issue, and as you said
maybe some devices will care for smaller sizes. So I think this is
depend on the devices' requirement, then can we add one boundary
parameter for the allocation according to the devices requirement? Or
like I said, we will waste lots of reserved memory if the granularity
is so large.
Thanks for your comments.
--
Baolin.wang
Best Regards
next prev parent reply other threads:[~2018-06-03 2:11 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-31 5:55 [PATCH 1/2] dma-coherent: Add one parameter to save available coherent memory Baolin Wang
2018-05-31 5:55 ` [PATCH 2/2] dma-coherent: Change the bitmap APIs Baolin Wang
2018-05-31 17:38 ` Robin Murphy
2018-06-03 2:11 ` Baolin Wang [this message]
2018-05-31 17:25 ` [PATCH 1/2] dma-coherent: Add one parameter to save available coherent memory Robin Murphy
2018-06-03 2:12 ` Baolin Wang
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