linux-kernel - Re: [PATCH v2 1/4] dma-mapping: Add devm_ interface for dma_map

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Message-id: <5392F63B.6050708@gmail.com>
Date:	Sat, 07 Jun 2014 14:23:39 +0300
From:	Eli Billauer <eli.billauer@...il.com>
To:	shuah.kh@...sung.com
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Joerg Roedel <joro@...tes.org>, devel@...verdev.osuosl.org,
	discuss@...-64.org, linux-kernel@...r.kernel.org,
	bhelgaas@...gle.com, iommu@...ts.linux-foundation.org,
	linux-pci@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 1/4] dma-mapping: Add devm_ interface for
 dma_map_single()

Hello Shuah,

We agree that the streaming API was originally *intended* for short 
map-unmap DMA sessions, and that dma_alloc_noncoherent() was the 
*intended* API for those who want to hold the DMA during a device's 
lifetime.

We also agree that on some platforms, DMA mappings are precious, and 
therefore any driver should unmap a region as soon as it's not needed 
anymore.

But if we stick to the citation you gave, it says "...unmapped right 
after it (unless you use dma_sync_* below)". So even in the streaming 
API's definition, there's an understanding, that the "streaming" DMA 
buffer can be held for more than a single session. And a good sync tool 
for that is made available.

Using cross-reference on Linux' code, I get a strong impression, that 
dma_alloc_NONcoherent() is pretty much unused (I counted 8 drivers). The 
streaming API's sync functions are heavily used, on the other hand. So 
one gets a hunch, that there's a lot of use of the streaming API in the 
kernel tree for long-term DMA mappings.

This wasn't the original intention -- we agree on that. But why is it 
wrong? Assuming that a driver needs to hold a DMA mapping for a long 
while, why does it matter if it was done with dma_alloc_noncoherent() or 
with dma_map_*()? They are equally wasteful, aren't they?

Why maintaining two API sets doing the same thing? Or is there a subtle 
functional difference I'm not aware of?

Thanks,
   Eli

On 06/06/14 20:02, Shuah Khan wrote:
>
> dma_map_single() and dma_unmap_single() are streaming DMA APIs. These
> are intended for one DMA transfer and unmapped right after it is done.
>
> dma buffers are limited shared resources for streaming that are
> shared by several drivers. Hence the need for use and release
> model.
>
> Please refer to the Using Streaming DMA mappings in DMA-API-HOWTO.txt
>
> "- Streaming DMA mappings which are usually mapped for one DMA
>   transfer, unmapped right after it (unless you use dma_sync_* below)
>   and for which hardware can optimize for sequential accesses.
>
>   This of "streaming" as "asynchronous" or "outside the coherency
>   domain".
>
>   Good examples of what to use streaming mappings for are:
>
>         - Networking buffers transmitted/received by a device.
>         - Filesystem buffers written/read by a SCSI device."
>
>
> If I understand your intended usage correctly, you are looking to
> allocate and hold the buffers for the lifetime of the driver. For
> such cases, dma_alloc_*() interfaces are the ones to use.
>
> Please also refer to DMA-API.txt as well. Hope this helps.
>
> -- Shuah
>
>

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