Re: [PATCH net] net: systemport: set dev->max_mtu to UMAC_MAX_MTU_SIZE

On Fri, Dec 18, 2020 at 12:54:33PM -0800, Florian Fainelli wrote:

On 12/18/20 12:52 PM, Vladimir Oltean wrote:

quoted

On Fri, Dec 18, 2020 at 12:30:20PM -0800, Florian Fainelli wrote:

quoted

On 12/18/20 12:24 PM, Vladimir Oltean wrote:

quoted

Hi Florian,

On Fri, Dec 18, 2020 at 09:38:43AM -0800, Florian Fainelli wrote:

quoted

The driver is already allocating receive buffers of 2KiB and the
Ethernet MAC is configured to accept frames up to UMAC_MAX_MTU_SIZE.

Fixes: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 drivers/net/ethernet/broadcom/bcmsysport.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/broadcom/bcmsysport.c b/drivers/net/ethernet/broadcom/bcmsysport.c
index 0fdd19d99d99..b1ae9eb8f247 100644
--- a/drivers/net/ethernet/broadcom/bcmsysport.c
+++ b/drivers/net/ethernet/broadcom/bcmsysport.c

@@ -2577,6 +2577,7 @@ static int bcm_sysport_probe(struct platform_device *pdev)
 			 NETIF_F_HW_VLAN_CTAG_TX;
 	dev->hw_features |= dev->features;
 	dev->vlan_features |= dev->features;
+	dev->max_mtu = UMAC_MAX_MTU_SIZE;
 
 	/* Request the WOL interrupt and advertise suspend if available */
 	priv->wol_irq_disabled = 1;

-- 
2.25.1

Do you want to treat the SYSTEMPORT Lite differently?

	/* Set maximum frame length */
	if (!priv->is_lite)
		umac_writel(priv, UMAC_MAX_MTU_SIZE, UMAC_MAX_FRAME_LEN);
	else
		gib_set_pad_extension(priv);

SYSTEMPORT Lite does not actually validate the frame length, so setting
a maximum number to the buffer size we allocate could work, but I don't
see a reason to differentiate the two types of MACs here.

And if the Lite doesn't validate the frame length, then shouldn't it
report a max_mtu equal to the max_mtu of the attached DSA switch, plus
the Broadcom tag length? Doesn't the b53 driver support jumbo frames?

And how would I do that without create a horrible layering violation in
either the systemport driver or DSA? Yes the b53 driver supports jumbo
frames.

Sorry, I don't understand where is the layering violation (maybe it doesn't
help me either that I'm not familiar with Broadcom architectures).

Is the SYSTEMPORT Lite always used as a DSA master, or could it also be
used standalone? What would be the issue with hardcoding a max_mtu value
which is large enough for b53 to use jumbo frames?