Subject: Re: [PATCH net] net/mlx4_en: Doorbell is byteswapped in Little Endian archs

From: Amir Vadai <[email-masked]>
Date: Mon, 22 Dec 2014 10:17:51 +0200

On 12/21/2014 9:53 PM, Sergei Shtylyov wrote:
> Hello.
Hi,

> On 12/21/2014 9:18 PM, Amir Vadai wrote:
> 
>> iowrite32() will byteswap it's argument on big endian archs.
>> iowrite32be() will byteswap on little endian archs.
>> Since we don't want to do this unnecessary byteswap on the fast path,
>> doorbell is stored in the NIC's native endianness. Using the right
>> iowrite() according to the arch endianness.
> 
>> CC: Wei Yang <[email-masked]>
>> CC: David Laight <[email-masked]>
>> Fixes: 6a4e812 ("net/mlx4_en: Avoid calling bswap in tx fast path")
>> Signed-off-by: Amir Vadai <[email-masked]>
>> ---
>>   drivers/net/ethernet/mellanox/mlx4/en_tx.c | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
>> index a308d41..6477cc7 100644
>> --- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
>> +++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
>> @@ -962,7 +962,16 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
>>   		tx_desc->ctrl.owner_opcode = op_own;
>>   		if (send_doorbell) {
>>   			wmb();
>> -			iowrite32(ring->doorbell_qpn,
>> +		/* Since there is no iowrite*_native() that writes the value
>> +		 * as is, without byteswapping - using the one the doesn't do
>> +		 * byteswapping in the relevant arch endianness.
>> +		 */
> 
>     Why the comment is not aligned with the code?
By mistake. Sending V1.

> 
>> +#if defined(__LITTLE_ENDIAN)
>> +			iowrite32(
>> +#else
>> +			iowrite32be(
>> +#endif
> 
>     Ugh...
Yes I think so too, but there is no iowrite*_native(). I plan to send a
patch to add it, but meanwhile the driver is completely broken in little
endian archs - so it must be fixed now. And just reverting to the old
behavior of bswap in the fast path looks like a bad alternative too.

> 
>> +				  ring->doorbell_qpn,
>>   				  ring->bf.uar->map + MLX4_SEND_DOORBELL);
> [...]
> 
> WBR, Sergei
> 
Amir