Re: [PATCH] net: stmmac: Allow zero for [tr]x_fifo_size

[Steven Price <steven.price@arm.com>]

[Moved Kunihiko to the To: line]

On 03/02/2025 10:38, Russell King (Oracle) wrote:
> On Mon, Feb 03, 2025 at 09:34:18AM +0000, Steven Price wrote:
>> Commit 8865d22656b4 ("net: stmmac: Specify hardware capability value
>> when FIFO size isn't specified") modified the behaviour to bail out if
>> both the FIFO size and the hardware capability were both set to zero.
>> However devices where has_gmac4 and has_xgmac are both false don't use
>> the fifo size and that commit breaks platforms for which these values
>> were zero.
>>
>> Only warn and error out when (has_gmac4 || has_xgmac) where the values
>> are used and zero would cause problems, otherwise continue with the zero
>> values.
>>
>> Fixes: 8865d22656b4 ("net: stmmac: Specify hardware capability value when FIFO size isn't specified")
>> Tested-by: Xi Ruoyao <xry111@xry111.site>
>> Signed-off-by: Steven Price <steven.price@arm.com>
> 
> I'm still of the opinion that the original patch set was wrong, and
> I was thinking at the time that it should _not_ have been submitted
> for the "net" tree (it wasn't fixing a bug afaics, and was a risky
> change.)
> 
> Yes, we had multiple places where we have code like:
> 
>         int rxfifosz = priv->plat->rx_fifo_size;
>         int txfifosz = priv->plat->tx_fifo_size;
> 
>         if (rxfifosz == 0)
>                 rxfifosz = priv->dma_cap.rx_fifo_size;
>         if (txfifosz == 0)
>                 txfifosz = priv->dma_cap.tx_fifo_size;
> 
>         /* Split up the shared Tx/Rx FIFO memory on DW QoS Eth and DW XGMAC */
>         if (priv->plat->has_gmac4 || priv->plat->has_xgmac) {
>                 rxfifosz /= rx_channels_count;
>                 txfifosz /= tx_channels_count;
>         }
> 
> and this is passed to stmmac_dma_rx_mode() and stmmac_dma_tx_mode().
> 
> We also have it in the stmmac_change_mtu() path for the transmit side,
> which ensures that the MTU value is not larger than the transmit FIFO
> size (which is going to fail as it's always done before or after the
> original patch set, and whether or not your patch is applied.)
> 
> Now, as for the stmmac_dma_[tr]x_mode(), these are method functions
> calling into the DMA code. dwmac4, dwmac1000, dwxgmac2, dwmac100 and
> sun8i implement methods for this.
> 
> Of these, dwmac4, dwxgmac2 makes use of the value passed into
> stmmac_dma_[tr]x_mode() - both of which initialise dma.[tr]x_fifo_size.
> dwmac1000, dwmac100 and sun8i do not make use of it.
> 
> So, going back to the original patch series, I still question the value
> of the changes there - and with your patch, it makes their value even
> less because the justification seemed to be to ensure that
> priv->plat->[tr]x_fifo_size contained a sensible value. With your patch
> we're going back to a situation where we allow these to effectively be
> "unset" or zero.
> 
> I'll ask the question straight out - with your patch applied, what is
> the value of the original four patch series that caused the breakage?
> 

I've no opinion whether the original series "had value" - I'm just 
trying to fix the breakage that entailed. My first attempt at a patch 
was indeed a (partial) revert, but Andrew was keen to find a better 
solution[1].

I'd prefer we don't delay getting a fix merged arguing about the finer 
details on this. Obviously once a fix is merged the code can be
improved at leisure. If you want to propose a straight revert then
by all means send the patch and I'll post a Tested-By.

Steve

[1] https://lore.kernel.org/all/fc08926d-b9af-428f-8811-4bfe08acc5b7@lunn.ch/