linux-kernel - Re: [PATCH 06/20] tile: support LSI MEGARAID SAS HBA hybrid dma

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Date:	Tue, 6 Aug 2013 13:00:06 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>
Subject: Re: [PATCH 06/20] tile: support LSI MEGARAID SAS HBA hybrid dma_ops

On 8/5/2013 4:52 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Aug 05, 2013 at 04:06:20PM -0400, Chris Metcalf wrote:
>> The LSI MEGARAID SAS HBA suffers from the problem where it can do
>> 64-bit DMA to streaming buffers but not to consistent buffers.
>> In other words, 64-bit DMA is used for disk data transfers and 32-bit
>> DMA must be used for control message transfers. According to LSI,
>> the firmware is not fully functional yet. This change implements a
>> kind of hybrid dma_ops to support this.
> If this is generic to LSI MegaRAID HBA shouldn't this change also be
> done for the other platforms?
>
> Or perhaps some other changes to make it work with other devices?

No, it's really specific to how tilegx maps the 64-bit PCI space, 32-bit PCI space, and memory.  I'll add more commentary to the git commit message to make this clearer.

>> @@ -573,6 +573,11 @@ int dma_set_coherent_mask(struct device *dev, u64 mask)
>>  	if (((dma_ops == gx_pci_dma_map_ops) ||
>>  	    (dma_ops == gx_legacy_pci_dma_map_ops)) &&
>>  	    (mask <= DMA_BIT_MASK(32))) {
>> +		if (dma_ops == gx_pci_dma_map_ops) {
>> +			dma_ops->alloc = tile_swiotlb_alloc_coherent;
>> +			dma_ops->free = tile_swiotlb_free_coherent;
>> +		}
>> +
> So.. that would change it for all of the devices on the host, not just
> for this specific one. That is not good is it?

Yes, good point; doing it this way might have some performance impact on other devices on the same host.  See following email for a revised change that updates the dma_ops pointer for that single device instead.

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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