linux-kernel - Re: [PATCH] arm: dma-mapping: move consistent_init to early

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Date:	Mon, 06 Dec 2010 22:22:15 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	dwalker@...eaurora.org, linux-arm-msm@...r.kernel.org,
	Nicolas Pitre <nico@...vell.com>, linux-kernel@...r.kernel.org,
	Jeff Ohlstein <johlstei@...eaurora.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Tejun Heo <tj@...nel.org>, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm: dma-mapping: move consistent_init to early_initcall

Russell King - ARM Linux wrote:
> On Fri, Dec 03, 2010 at 12:06:53PM -0800, Saravana Kannan wrote:
>> The MSM8660 SoC uses the TrustZone technology and the Linux kernel  
>> executes in normal/non-secure domain. When the second core is brought  
>> out of reset, it starts executing a secure image which then jumps to  
>> "secondary_startup". So, before bringing the second core out of reset,  
>> we need to inform the secure domain code where secondary_startup is  
>> located in memory.
>>
>> We do the communication with the secure code by using buffers in memory.  
>>  The cache treats the NS (non secure) bit as an additional address bit  
>> when tagging memory. Hence, cache accesses are not coherent between the  
>> secure and non-secure domains. So, the secure side flushes it's cache  
>> after writing to the buffer. To properly read the response from the  
>> secure side, the kernel has to pick a buffer that isn't cacheable in the  
>> first place. We have similar issues in the reverse direction.
> 
> So when ARM gets DMA-coherent caches, you of course aren't going to
> complain that the DMA APIs start avoiding doing the current tricks with
> non-cacheable memory?
> 
> I view what you're doing above with the DMA API as an abuse of the API.
> Just like the problems we're facing with ioremap() being used on system
> RAM, you're asking for problems when the ARM architecture changes because
> you're using an API for it's current properties, not for its purpose.

You are right. Thanks for catching this.

So, that basically leaves us with these options:
* Create another API to allow getting uncached pages. I don't think we 
will be the first or the last to want uncached pages. Even if ARM 
introduces DMA-coherent caches, it's  possible for SoC vendors to have 
other h/w blocks that could directly operate on memory. The cache might 
not be coherent with these h/w blocks.
* Add a cache invalidate API that's outside the DMA APIs and can be used 
  when needed.

Do one of the above two options sound reasonable to you?

> I've been on for years about purpose-designed APIs for cache issues,
> and every time someone abuses them, they eventually end up suffering
> breakage.
> 
> Let's wait until the full set of patches is available before discussing
> further.

Jeff Ohlstein sent out a series of patches ([PATCH 0/5] SMP support for 
msm). The patch that deals with talking to the secure domain code is 
titled "[PATCH 2/5] msm: scm-boot: Support for setting cold/warm boot 
addresses". I see that you replied to an email on that, but it's not 
clear if you connected that patch with this thread.

Thanks,
Saravana

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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