netdev - Re: [PATCH net v4 3/3] net: stmmac: Specify hardware capability value when FIFO size isn't specified

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Message-ID: <2ed9e7c7-e760-409e-a431-823bc3f21cb7@lunn.ch>
Date: Fri, 31 Jan 2025 16:29:59 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Steven Price <steven.price@....com>
Cc: Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jose Abreu <joabreu@...opsys.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Russell King <linux@...linux.org.uk>,
	Yanteng Si <si.yanteng@...ux.dev>, Furong Xu <0x1207@...il.com>,
	Joao Pinto <Joao.Pinto@...opsys.com>, netdev@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v4 3/3] net: stmmac: Specify hardware capability
 value when FIFO size isn't specified

On Fri, Jan 31, 2025 at 03:03:11PM +0000, Steven Price wrote:
> On 31/01/2025 14:47, Andrew Lunn wrote:
> >>> I'm guessing, but in your setup, i assume the value is never written
> >>> to a register, hence 0 is O.K. e.g. dwmac1000_dma_operation_mode_rx(),
> >>> the fifosz value is used to determine if flow control can be used, but
> >>> is otherwise ignored.
> >>
> >> I haven't traced the code, but that fits my assumptions too.
> > 
> > I could probably figure it out using code review, but do you know
> > which set of DMA operations your hardware uses? A quick look at
> > dwmac-rk.c i see:
> > 
> >         /* If the stmmac is not already selected as gmac4,
> >          * then make sure we fallback to gmac.
> >          */
> >         if (!plat_dat->has_gmac4)
> >                 plat_dat->has_gmac = true;
> 
> has_gmac4 is false on this board, so has_gmac will be set to true here.

Thanks. Looking in hwif.c, that means dwmac1000_dma_ops are used.

dwmac1000_dma_operation_mode_rx() just does a check:

	if (rxfifosz < 4096) {
		csr6 &= ~DMA_CONTROL_EFC;

but otherwise does not use the value.

dwmac1000_dma_operation_mode_tx() appears to completely ignore fifosz.

So i would say all zero is valid for has_gmac == true, but you might
gain flow control if a value was passed.

A quick look at dwmac100_dma_operation_mode_tx() suggests fifosz is
also ignored, and dwmac100_dma_operation_mode_rx() does not exist. So
all 0 is also valid for gmac == false, gmac4 == false, and xgmac ==
false.

dwmac4_dma_rx_chan_op_mode() does use the value to determine mtl_rx_op
which gets written to a register. So gmac4 == true looks to need
values. dwxgmac2_dma_rx_mode() is the same, so xgmac = true also need
valid values.

Could you cook up a fix based on this?

	Andrew