linux-kernel - Re: [PATCH v3 4/7] of: configure the platform device dma parameters

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Date:	Tue, 6 May 2014 16:44:40 -0400
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Rob Herring <robherring2@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Russell King <linux@....linux.org.uk>,
	Olof Johansson <olof@...om.net>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"Strashko, Grygorii" <grygorii.strashko@...com>
Subject: Re: [PATCH v3 4/7] of: configure the platform device dma parameters

On Tuesday 06 May 2014 05:40 AM, Arnd Bergmann wrote:
> On Monday 05 May 2014 17:47:32 Santosh Shilimkar wrote:
> 
>> +       dev->coherent_dma_mask = DMA_BIT_MASK(32);
>> +       if (!dev->dma_mask)
>> +               dev->dma_mask = &dev->coherent_dma_mask;
>> +
>> +       /*
>> +        * if dma-ranges property doesn't exist - just return else
>> +        * setup the dma offset
>> +        */
>> +       ret = of_dma_get_range(dev->of_node, &dma_addr, &paddr, &size);
>> +       if (ret < 0) {
>> +               dev_dbg(dev, "no dma range information to setup\n");
>> +               return;
>> +       }
>> +
>> +       /* DMA ranges found. Calculate and set dma_pfn_offset */
>> +       dev->dma_pfn_offset = PFN_DOWN(paddr - dma_addr);
>> +       dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset);
>> +}
> 
> I think there should at least be a comment about why we are computing
> the correct DMA mask here and then ignore that and just use DMA_BIT_MASK(32)
> instead. I understand that Russell and Rob prefer it that way and I'm not
> going to argue, but I find it counterintuitive and I think it deserves
> an explanation in the source code for anybody who is trying to figure
> out how things fit together.
> 
In this patch, the dma_mask related code is just moved. We are not calculating
dma_mask either. I was looking for the history of how DMA_BIT_MASK(32)
landed up but couldn't trace it down apart from the fact that the code was
carried from powerPC. May be Rob knows.

How about below comment ? I didn't delibratly added point about bus
intercepting drivers dma_set_*mask() call etc.

/*
 * Set default dma-mask to 32 bit. Drivers are expected to setup
 * the correct supported dma_mask.
 */  
+       dev->coherent_dma_mask = DMA_BIT_MASK(32);
+       if (!dev->dma_mask)
+               dev->dma_mask = &dev->coherent_dma_mask;

Regards,
Santosh
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