Messages in this thread | | | Subject | Re: [PATCH V2 1/3] scsi: mptxsas: try 64 bit DMA when 32 bit DMA fails | From | Sinan Kaya <> | Date | Mon, 9 Nov 2015 09:07:36 -0500 |
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On 11/9/2015 3:59 AM, Arnd Bergmann wrote: > On Monday 09 November 2015 08:09:39 Hannes Reinecke wrote: >> On 11/09/2015 02:57 AM, Sinan Kaya wrote: >>> Current code gives up when 32 bit DMA is not supported. >>> This problem has been observed on systems without any >>> memory below 4 gig. >>> >>> This patch tests 64 bit support before bailing out to find >>> a working combination. >>> >> That feels decidedly odd. >> >> Why do you probe for 64bit if 32bit fails? > > 32-bit DMA mask on PCI cannot fail, we rely on that in all sorts > of places. If the machine has all of its RAM visible above 4GB > PCI bus addresses, it must use an IOMMU.
Can you be specific? PCIe does not have this limitation. It supports 32 bit and 64 bit TLPs.
I have not seen any limitation so far in the OS either.
Using IOMMU is fine but not required if the endpoint is a true 64 bit supporting endpoint. This endpoint supports 64bit too.
> >> Typically it's the other way round, on the grounds that 64bit DMA >> should be preferred over 32bit. >> Can you explain why it needs to be done the other way round here? > > Something else is odd here, the driver already checks for > dma_get_required_mask(), which will return the smallest mask > that fits all of RAM. If the machine has any memory above 4GB, > it already uses the 64-bit mask, and only falls back to > the 32-bit mask if that fails or if all memory fits within the > first 4GB. >
I'll add some prints in the code to get to the bottom of it. I think the code is checking more than just the required mask and failing in one of the other conditions. At least that DMA comparison code was more than what I have ever seen.
> So both the description and the patch are wrong. :( > > Arnd >
-- Sinan Kaya Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
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