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Re: linux cache routines for Write-back cache policy on MIPS24KE

To: veerasena reddy <veerasena_b@yahoo.co.in>
Subject: Re: linux cache routines for Write-back cache policy on MIPS24KE
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 1 Oct 2007 11:59:54 +0100
Cc: linux-mips <linux-mips@linux-mips.org>, "linux-kernel.org" <linux-kernel@vger.kernel.org>
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On Mon, Oct 01, 2007 at 10:04:32AM +0100, veerasena reddy wrote:

> I have ported Linux-2.6.18 kernel on MIPS24KE
> processor. I am using write back cache policy.
> 
> Could you please guide me under what cases the below
> cache API's are being used:
> - dma_cache_wback_inv() : Could you explain  what
> exactly this function does
> - dma_cache_wback() : This function write back the
> cache data to memory
> - dma_cache_inv  : This function invalidate the cache
> tags. so subsequent access will fetch from memory.
> 
> Once I looked the above function definitions in
> linux-2.6.18/arch/mips/mm/c-r4k.c.
> All these function's implemetation are same except
> bc_wbak_inv() is called in both dma_cache_wback-inv()
> and dma_cache_wback(), where as bc_inv() is called in
> case of dma_cache_inv.
> 
> Also, bc_inv()/bc_wbak_inv are define as null
> implementation for R4000.
> That means all three functions are doing same
> functionality in case of R4000.
> 
> What are the difference between these three functions.
> Under what cases these functions are used. 

An internal only interface to be used with I/O cache coherency.

> Please guide me if you have any links which will
> explain these API's.

Easy answer, don't use them, for 2.6.24 I've queued a patch to kill this
API.  Documentation/DMA-API.txt documents how to properly deal with I/O
coherency in Linux.

  Ralf

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