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

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Date:   Wed, 22 Aug 2018 18:40:27 +0000
From:   Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
To:     Vineet Gupta <Vineet.Gupta1@...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@...opsys.com" <Alexey.Brodkin@...opsys.com>
Subject: Re: [PATCH v2 2/4] ARC: allow to use IOC and non-IOC DMA devices
 simultaneously

Hi Vineet,

On Mon, 2018-08-20 at 15:34 -0700, Vineet Gupta wrote:
> 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.
Do you mean comment in commit description or comment right in sources?

> 
> > > > +	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.

Just to be sure that we understand both each other and source code correctly: 
- In coherent case we use dma_direct_* ops which doesn't use ARC alloc functions (arch_dma_{alloc|free})
- In non-coherent case we use dma_noncoherent_* ops which uses ARC alloc functions (arch_dma_{alloc|free})

> 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 ?

DMA_ATTR_NON_CONSISTENT flag is not related to IOC topic.
It is a flag which we can pass to dma_{alloc|free}_attrs function from driver side.

According to documentation:
  DMA_ATTR_NON_CONSISTENT: Lets the platform to choose to return either
  consistent or non-consistent memory as it sees fit.

We check this flag in arch_dma_alloc (which are used in non-coherent case) to
skip MMU mapping if we are advertised that consistency is not required.

So, actually we can get rid of this flag checking in arch_dma_alloc and 
simply always do MMU mapping to enforce non-cachability and return
non-cacheable memory even if DMA_ATTR_NON_CONSISTENT is passed.
But I don't sure we want to do that.

BTW: dma_alloc_coherent is simply dma_alloc_attrs with attrs == 0.

-- 
 Eugeniy Paltsev