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

Santosh Shilimkar
2014-05-06T20:45:26Z

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