netdev - Re: [PATCH net-next] net/mlx4_en: Advertise HIGH DMA support only if dma mask is 64 bit

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Date:	Tue, 13 Aug 2013 14:19:28 +0300
From:	Amir Vadai <amirv@...lanox.com>
To:	Ben Hutchings <bhutchings@...arflare.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	<netdev@...r.kernel.org>, Or Gerlitz <ogerlitz@...lanox.com>,
	Alexander Guller <alexg@...lanox.com>
Subject: Re: [PATCH net-next] net/mlx4_en: Advertise HIGH DMA support only
 if dma mask is 64 bit

On 12/08/2013 19:32, Ben Hutchings wrote:
> On Mon, 2013-08-12 at 11:30 +0300, Amir Vadai wrote:
>> Need to check that the pci device DMA mask was set to 64 bit support
>> (done by mlx4_core) before advertising HIGH DMA support in higher drivers.
>> Otherwise, may run into an issue that skb/frag/data is in high memory
>> and can't dma map it, thus packets are dropped.
> 
> NETIF_F_HIGHDMA means that the driver can transmit skbs with page
> fragments in high memory, i.e. it doesn't require them to have an
> existing virtual mapping for the kernel.  If I'm not mistaken, this
> should be cleared only for drivers that don't use DMA or kmap functions.
> 
> On a 64-bit machine there is no highmem and without NETIF_F_HIGHDMA you
> can still be given pages at any physical address.
> 
> On a 32-bit machine highmem usually starts at 1GB and I doubt you have
> any chips that are that limited in DMA range. :-)
> 
> I really doubt this is the fix you're looking for.
> 
You are right. We were mixing highmem with addresses greater than 2^32.
Thanks for reviewing :)

Dave, please abandon this patch.

Amir.

> Ben.
> 
>> Signed-off-by: Alexander Guller <alexg@...lanox.com>
>> Signed-off-by: Amir Vadai <amirv@...lanox.com>
>> ---
>>  drivers/net/ethernet/mellanox/mlx4/en_netdev.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
>> index fa37b7a..9568b77 100644
>> --- a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
>> +++ b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
>> @@ -2317,9 +2317,13 @@ int mlx4_en_init_netdev(struct mlx4_en_dev *mdev, int port,
>>  	dev->vlan_features = dev->hw_features;
>>  
>>  	dev->hw_features |= NETIF_F_RXCSUM | NETIF_F_RXHASH;
>> -	dev->features = dev->hw_features | NETIF_F_HIGHDMA |
>> +	dev->features = dev->hw_features |
>>  			NETIF_F_HW_VLAN_CTAG_TX | NETIF_F_HW_VLAN_CTAG_RX |
>>  			NETIF_F_HW_VLAN_CTAG_FILTER;
>> +
>> +	if (mdev->dev->pdev->dma_mask == DMA_BIT_MASK(64))
>> +		dev->features |= NETIF_F_HIGHDMA;
>> +
>>  	dev->hw_features |= NETIF_F_LOOPBACK;
>>  
>>  	if (mdev->dev->caps.steering_mode ==
> 

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