Re: [PATCH net] net: stmmac: Guard against txfifosz=0

[Florian Fainelli <f.fainelli@gmail.com>]

On 4/12/2020 11:27 AM, Jakub Kicinski wrote:
> On Sat, 11 Apr 2020 20:49:31 -0700 Florian Fainelli wrote:
>> After commit bfcb813203e619a8960a819bf533ad2a108d8105 ("net: dsa:
>> configure the MTU for switch ports") my Lamobo R1 platform which uses
>> an allwinner,sun7i-a20-gmac compatible Ethernet MAC started to fail
>> by rejecting a MTU of 1536. The reason for that is that the DMA
>> capabilities are not readable on this version of the IP, and there is
>> also no 'tx-fifo-depth' property being provided in Device Tree. The
>> property is documented as optional, and is not provided.
>>
>> The minimum MTU that the network device accepts is ETH_ZLEN - ETH_HLEN,
>> so rejecting the new MTU based on the txfifosz value unchecked seems a
>> bit too heavy handed here.
> 
> OTOH is it safe to assume MTUs up to 16k are valid if device tree lacks
> the optional property? Is this change purely to preserve backward
> (bug-ward?) compatibility, even if it's not entirely correct to allow
> high MTU values? (I think that'd be worth stating in the commit message
> more explicitly.) Is there no "reasonable default" we could select for
> txfifosz if property is missing?

Those are good questions, and I do not know how to answer them as I am
not familiar with the stmmac HW design, but I am hoping Jose can respond
on this patch. It does sound like providing a default TX FIFO size would
solve that MTU problem, too, but without a 'tx-fifo-depth' property
specified in Device Tree, and with the "dma_cap" being empty for this
chip, I have no idea what to set it to.

> 
>> Fixes: eaf4fac47807 ("net: stmmac: Do not accept invalid MTU values")
>> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
>> ---
>>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> index e6898fd5223f..9c63ba6f86a9 100644
>> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> @@ -3993,7 +3993,7 @@ static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
>>  	new_mtu = STMMAC_ALIGN(new_mtu);
>>  
>>  	/* If condition true, FIFO is too small or MTU too large */
>> -	if ((txfifosz < new_mtu) || (new_mtu > BUF_SIZE_16KiB))
>> +	if ((txfifosz < new_mtu && txfifosz) || (new_mtu > BUF_SIZE_16KiB))
>>  		return -EINVAL;
>>  
>>  	dev->mtu = new_mtu;
> 

-- 
Florian