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

[Chen-Yu Tsai]

On Mon, Apr 13, 2020 at 2:42 PM Jose Abreu <Jose.Abreu@xxxxxxxxxxxx> wrote:
>
> From: Florian Fainelli <f.fainelli@xxxxxxxxx>
> Date: Apr/12/2020, 19:31:55 (UTC+00:00)
>
> >
> >
> > 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.
>
> Unfortunately, allwinner uses GMAC which does not have any mean to detect
> TX FIFO Size. Default value in HW is 2k but this can not be the case in
> these platforms if HW team decided to change it.

I looked at all the publicly available datasheets and Allwinner uses
4K TX FIFO and 16K RX FIFO in all SoCs. Not sure if this would help.

ChenYu

> Anyway, your patch looks sane to me. But if you start seeing TX Queue
> Timeout for higher MTU values then you'll need to add tx-fifo-depth
> property.
>
> ---
> Thanks,
> Jose Miguel Abreu
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