Re: [PATCH v6 6/7] PCI: update dma configuration from DT

[Murali Karicheri <m-karicheri2-l0cyMroinI0@public.gmane.org>]

On 02/24/2015 08:53 PM, Bjorn Helgaas wrote:
> On Thu, Feb 05, 2015 at 04:52:58PM -0500, Murali Karicheri wrote:
>> If there is a DT node available for the root bridge's parent device,
>> use the dma configuration from that device node. For example, keystone
>> PCI devices would require dma_pfn_offset to be set correctly in the
>> device structure of the pci device in order to have the correct dma mask.
>> The DT node will have dma-ranges defined for this. Also support using
>> the DT property dma-coherent to allow coherent DMA operation by the
>> PCI device.
>
> Hi Murali,
>
> I'm guessing this is the patch that actually fixes things for Keystone.
>
Yes, but depends on other patches in the series.

> And it looks like it should also fix things for other hardware that has
> similar characteristics.

This originally started as a patch for Keystone, but modified to apply 
for other platforms as well. Tested by one another platform AFAIK, by 
Suravee Suthikulpanit on AMD Seattle platform w/ PCI Generic Host 
Controller. See the Tested-By from him against this series.

  So I'd like to include some higher-level text
> about that here.  For example:
>
>    This fixes DMA on systems where DMA addresses are a constant offset
>    from CPU physical addresses.
>

That is one fix. It also update dma configuration for the device
such as dma operations through a call to of_dma_configure() in 5/7.
You may add the above description in 5/7. I didn't add it in the 
description because of_dma_configure() API call takes care of it already.

> (I don't know exactly how these patches all fit together, so that's
> probably not accurate, but that's the *sort* of thing I'd like to include.)
>
> If that actually *is* what's going on, I have to wonder why this isn't
> implemented as a very simple IOMMU instead of adding dma_pfn_offset,
> which is present on all arches but only used on ARM.  In some sense that
> offset is parallel but incompatible with an IOMMU: they both translate DMA
> addresses into system RAM addresses.

I don't have much history on any previous discussion on the subject you 
are referring to. I assume it would have happened when 
of_dma_configure() was first introduced. On Keystone, we don't have 
IOMMU support and dma_pfn_offset is needed to translate DMA address to 
System RAM address and vice-versa. So this has to be supported for 
Keystone. There can be enhancement for IOMMU with out impacting this 
feature for Keystone.

I know that Will Deacon is working on updates for IOMMU which is 
partially touched by my series (1/7). Probably this can be a question 
when that patches comes up for review or could be a seperate discussion 
topic.

I think it is better this series is applied to the next branch for v4.1 
as soon as possible so that others can add features on top of this 
without breaking the function for Keystone.

I am looking forward for the merge of this series to the next branch at 
the earliest.

Thanks.

Murali
>
> I know you're not adding this, and I assume somebody explored that option
> and rejected it for good reasons.  I just missed that discussion.
>
>> This patch use the new helper function of_pci_dma_configure() to update
>> the device dma configuration.
>>
>> Cc: Joerg Roedel<joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>
>> Cc: Grant Likely<grant.likely-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
>> Cc: Rob Herring<robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
>> Cc: Will Deacon<will.deacon-5wv7dgnIgG8@public.gmane.org>
>> Cc: Russell King<linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>
>> Cc: Arnd Bergmann<arnd-r2nGTMty4D4@public.gmane.org>
>> Cc: Suravee Suthikulpanit<Suravee.Suthikulpanit-5C7GfCeVMHo@public.gmane.org>
>>
>> Acked-by: Bjorn Helgaas<bhelgaas-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>> Signed-off-by: Murali Karicheri<m-karicheri2-l0cyMroinI0@public.gmane.org>
>> ---
>>   drivers/pci/probe.c |    2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index 23212f8..d7dcd6c 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -6,6 +6,7 @@
>>   #include<linux/delay.h>
>>   #include<linux/init.h>
>>   #include<linux/pci.h>
>> +#include<linux/of_pci.h>
>>   #include<linux/pci_hotplug.h>
>>   #include<linux/slab.h>
>>   #include<linux/module.h>
>> @@ -1520,6 +1521,7 @@ void pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
>>   	dev->dev.dma_mask =&dev->dma_mask;
>>   	dev->dev.dma_parms =&dev->dma_parms;
>>   	dev->dev.coherent_dma_mask = 0xffffffffull;
>> +	of_pci_dma_configure(dev);
>>
>>   	pci_set_dma_max_seg_size(dev, 65536);
>>   	pci_set_dma_seg_boundary(dev, 0xffffffff);
>> --
>> 1.7.9.5
>>
>> --
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-- 
Murali Karicheri
Linux Kernel, Texas Instruments