Subject: Re: [E1000-devel] [PATCH] e100: Add missing dma sync for proper operation with non-coherent caches.

From: David Daney <[email-masked]>
Date: Tue, 26 Aug 2008 09:54:47 -0700

Kok, Auke wrote:
> David Daney wrote:
>> I am running the e100 driver on a MIPS 4KEc system (32 bit mips with
>> non-coherent DMA).  There was a problem where received packets would
>> get 'stuck' for several seconds at a time and then be released all at
>> once.
>>
>> The cause was that if an interrupt were received when no RX packets
>> were available, the status for the receive buffer would be stuck in
>> the cache, so when the next interrupt arrived the old status value was
>> read (indicating no packets available) instead of the new value.
>>
>> The fix is to call pci_dma_sync_single_for_device on the RX if the
>> packet is not available to invalidate the cache so that at the next
>> interrupt valid status is returned.
>>
>> The driver currently calls pci_dma_sync_single_for_cpu before reading
>> the status, and this is indeed needed for cases like the R10000 CPU
>> where the cache can be polluted by speculative execution, but for most
>> machines it is a nop.
>>
>> The patch was tested on 2.6.17-rc4 on a MIPS 4KEc.
> 
> 
> lol, that's a bit old :)
> 

It was a typo.  The real version is 2.6.27-rc4 (aka the HEAD).

The bug is present on the HEAD.  Sorry for the confusion.

David Daney.

> A LOT of work has gone into 2.6.26+'s version of e100 that addresses specifically
> non-coherent DMA machines, including a patch that looks very close to what you
> write below here.
> 
> can you test the latest git tree and see if you still need (a modified or updated
> version) of your patch?
> 
> I know for sure that the guys maintaining e100 do not have anything close to your
> system, so they can't test that.
> 
> Cheers,
> 
> Auke
> 
>> Signed-off-by: David Daney <[email-masked]>
>> ---
>>  drivers/net/e100.c |    5 +++++
>>  1 files changed, 5 insertions(+), 0 deletions(-)
>>
>> diff --git a/drivers/net/e100.c b/drivers/net/e100.c
>> index 19d32a2..fb8d551 100644
>> --- a/drivers/net/e100.c
>> +++ b/drivers/net/e100.c
>> @@ -1840,6 +1840,11 @@ static int e100_rx_indicate(struct nic *nic, struct rx *rx,
>>  
>>  			if (readb(&nic->csr->scb.status) & rus_no_res)
>>  				nic->ru_running = RU_SUSPENDED;
>> +		/* We are done looking at the buffer.  Prepare it for
>> +		 * more DMA.  */
>> +		pci_dma_sync_single_for_device(nic->pdev, rx->dma_addr,
>> +					       sizeof(struct rfd),
>> +					       PCI_DMA_FROMDEVICE);
>>  		return -ENODATA;
>>  	}
>>  
>