netdev - Re: [PATCH] ravb: add support for changing MTU

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Message-ID: <e946b694-a241-b1fa-a383-3edfe35468e5@cogentembedded.com>
Date:   Wed, 14 Feb 2018 14:34:09 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Niklas Söderlund <niklas.soderlund@...natech.se>
Cc:     "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH] ravb: add support for changing MTU

Hello!

On 02/13/2018 04:12 PM, Niklas Söderlund wrote:

>> On 02/12/2018 11:00 PM, Niklas Söderlund wrote:
>>
>>> Allow for chancing the MTU within the limit of the maximum size of a
>>
>>    Changing. :-)
> 
> Yes :-)
> 
>>> descriptor (2048 bytes). Add the callback to change MTU from user-space
>>> and take the configurable MTU into account when configuring the
>>> hardware.
>>>
>>> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
>> [...]
>>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
>>> index c87f57ca44371586..a4870c9e42195802 100644
>>> --- a/drivers/net/ethernet/renesas/ravb_main.c
>>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>>> @@ -300,9 +300,9 @@ static void ravb_ring_format(struct net_device *ndev, int q)
>>>  	for (i = 0; i < priv->num_rx_ring[q]; i++) {
>>>  		/* RX descriptor */
>>>  		rx_desc = &priv->rx_ring[q][i];
>>> -		rx_desc->ds_cc = cpu_to_le16(PKT_BUF_SZ);
>>> +		rx_desc->ds_cc = cpu_to_le16(priv->rx_buf_sz);
>>>  		dma_addr = dma_map_single(ndev->dev.parent, priv->rx_skb[q][i]->data,
>>> -					  PKT_BUF_SZ,
>>> +					  le16_to_cpu(rx_desc->ds_cc),
>>
>>   Why not 'priv->rx_buf_sz'?
> 
> To align the arguments used with the one in ravb_rx() which uses 
> le16_to_cpu(rx_desc->ds_cc) already before this patch.

   Why?

> 	static bool ravb_rx(struct net_device *ndev, int *quota, int q)
> 	{
> 	...
> 	    /* Refill the RX ring buffers. */
> 	    for (; priv->cur_rx[q] - priv->dirty_rx[q] > 0; priv->dirty_rx[q]++) {
> 		    ...
> 		    desc->ds_cc = cpu_to_le16(priv->rx_buf_sz);
> 
> 		    if (!priv->rx_skb[q][entry]) {
> 			    ...
> 			    dma_addr = dma_map_single(ndev->dev.parent, skb->data,
> 						      le16_to_cpu(desc->ds_cc),
> 						      DMA_FROM_DEVICE);
> 			    ...
> 		    }    
> 		    ...
> 	    }
> 	    ...
> 	}
> 
> I have no preference one way or the other but I think both call sites 
> should look the same :-)

   Why? I don't like this idea at all...

>> [...]
>>> @@ -346,6 +346,10 @@ static int ravb_ring_init(struct net_device *ndev, int q)
>>>  	int ring_size;
>>>  	int i;
>>>  
>>> +	/* +16 gets room from the status from the card. */
>>> +	priv->rx_buf_sz = (ndev->mtu <= 1492 ? PKT_BUF_SZ : ndev->mtu) +
>>> +		ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN + 16;
>>
>>    Mhm, I don't think FCS gets added to the frame buffer...

   It certainly isn't included, judging by the manuals... Instead 2-byte checksum is
included after the frame data (if checksumming is enabled).

> And why add 16?
> 
> And +16 is added as the comment above states, to leave from the 
> descriptor status appended by the hardware.

   I don't see any appended status in the manuals, do you?

> This is already the case 
> with PKT_BUF_SZ which for ravb is is set to 1538. MTU (1500) + ETH_HLEN 
> (14) + VLAN_HLEN(4) + ETH_FCS_LEN(4) + ravb status (16) == 1538.

> This is also what the sh_eth driver do and I think it's value to keep 
> these to driver as similar as possible in this regard, would you not 

  The DMA hardware is totally different, so I don't see any value in mirroring what sh_eth does...

> agree? If in deed the FSC is not added I think we should fix this for 
> both drivers in a follow up commit.

   Probably a good idea... :-)

[...]

MBR, Sergei