linux-kernel - Re: [PATCH v2 2/4] ARC: allow to use IOC and non-IOC DMA devices simultaneously

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Date:   Mon, 20 Aug 2018 15:34:31 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "hch@....de" <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Subject: Re: [PATCH v2 2/4] ARC: allow to use IOC and non-IOC DMA devices
 simultaneously

On 08/13/2018 10:08 AM, Eugeniy Paltsev wrote:
> On Mon, 2018-08-13 at 16:24 +0000, Vineet Gupta wrote:
>> On 07/30/2018 09:26 AM, Eugeniy Paltsev wrote:
>>> @@ -1263,11 +1254,7 @@ void __init arc_cache_init_master(void)
>>>  	if (is_isa_arcv2() && ioc_enable)
>>>  		arc_ioc_setup();
>>>  
>>> -	if (is_isa_arcv2() && ioc_enable) {
>>> -		__dma_cache_wback_inv = __dma_cache_wback_inv_ioc;
>>> -		__dma_cache_inv = __dma_cache_inv_ioc;
>>> -		__dma_cache_wback = __dma_cache_wback_ioc;
>>> -	} else if (is_isa_arcv2() && l2_line_sz && slc_enable) {

For the casual reader I'd add a comment why this was deleted.

>>> +	if (is_isa_arcv2() && l2_line_sz && slc_enable) {
>>>  		__dma_cache_wback_inv = __dma_cache_wback_inv_slc;
>>>  		__dma_cache_inv = __dma_cache_inv_slc;
>>>  		__dma_cache_wback = __dma_cache_wback_slc;

[snip]

>>>  
>>> -	/*
>>> -	 * IOC relies on all data (even coherent DMA data) being in cache
>>> -	 * Thus allocate normal cached memory
>>> -	 *
>>> -	 * The gains with IOC are two pronged:
>>> -	 *   -For streaming data, elides need for cache maintenance, saving
>>> -	 *    cycles in flush code, and bus bandwidth as all the lines of a
>>> -	 *    buffer need to be flushed out to memory
>>> -	 *   -For coherent data, Read/Write to buffers terminate early in cache
>>> -	 *   (vs. always going to memory - thus are faster)
>>> -	 */
>>> -	if ((is_isa_arcv2() && ioc_enable) ||
>>> -	    (attrs & DMA_ATTR_NON_CONSISTENT))
>>> +	if (attrs & DMA_ATTR_NON_CONSISTENT)
>>>  		need_coh = 0;
>>>  

[snip]

> Yep, I tested that.
> And it works fine with both @ioc_enable == 0 and @ioc_enable == 1
> Note that we check this variable in arch_setup_dma_ops() function now.
> 
> So this arch_dma_{alloc,free} are used ONLY in case of software assisted cache maintenance.
> That's why we had to do MMU mapping to enforce non-cachability regardless of @ioc_enable.

Reading kernel/dma/* I see what you mean. We check @ioc_enable at the time of
registering the dma op for coherent vs. non coherent case, so there's common vs.
ARC versions of alloc/free for coherent vs. noncoherent. But then I'm curious why
do we bother to check the following in new arch_dma_(alloc|free) at all.

	if (attrs & DMA_ATTR_NON_CONSISTENT)

Isn't it supposed to be NON_CONSISTENT always given the way new code works ?