linux-kernel - Re: [PATCH 5/6] swiotlb: Make swiotlb bookkeeping functions visible in the header file.

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Date:	Wed, 19 May 2010 07:22:54 +0200
From:	Albert Herranz <albert_herranz@...oo.es>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	konrad.wilk@...cle.com, Ian.Campbell@...citrix.com,
	jeremy@...p.org, linux-kernel@...r.kernel.org, chrisw@...s-sol.org,
	iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
	linux@....linux.org.uk
Subject: Re: [PATCH 5/6] swiotlb: Make swiotlb bookkeeping functions visible
 in the header file.

On 05/19/2010 05:34 AM, FUJITA Tomonori wrote:
>> Every single KB counts on the Wii. It has just 24MB of MEM1 and 64MB of MEM2 (discontiguous memory ranges).
>> I'm using 1MB for the SWIOTLB for now, but of course that can be further tweaked down.
> 
> You can decrease the swiotlb memory however you can't fix the root
> problems of swiotlb:
> 
> - it needs pre-allocated memory
> - it can't handle the out-of-pre-allocated memory situation.
> 

Yes, agreed.
Although having a dedicated pool _may_ be an advantage on a memory constrained system: it can guarantee swiotlb allocations (as long as the pool has been properly sized) even when the system is using up all memory.

> I mean that Wii doesn't boot on the root device on the USB controller,
> right?
> 

The current "bootloaders" boot images from the external SD card.
But Linux itself can have its root device on a USB storage device.

>> I posted (in the past) a patch series [1] in which I made the
>> dmabounce code in the ARM architecture tree available to other
>> architectures, and used that to implement the needed bouncing
>> infrastructure.  But I was told then by Russell to use swiotlb
>> instead [2].
> 
> I think that we need to modify swiotlb for embedded archs. Otherwise,
> I don't think that swiotlb can replace arm's dmabounce.
> 
> Ok, I'll implement something like that.
> 

Thanks. Looking forward to it :)

Cheers,
Albert
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