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

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Message-ID: <4e98f967-f636-46fb-9eca-d383b9495b86@roeck-us.net>
Date: Sat, 1 Feb 2025 11:14:41 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
Cc: 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

Hi,

On Mon, Jan 27, 2025 at 10:38:20AM +0900, Kunihiko Hayashi wrote:
> When Tx/Rx FIFO size is not specified in advance, the driver checks if
> the value is zero and sets the hardware capability value in functions
> where that value is used.
> 
> Consolidate the check and settings into function stmmac_hw_init() and
> remove redundant other statements.
> 
> If FIFO size is zero and the hardware capability also doesn't have upper
> limit values, return with an error message.
> 
> Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>

This patch breaks qemu's stmmac emulation, for example for
npcm750-evb. The error message is:
	stmmaceth f0804000.eth: Can't specify Rx FIFO size

The setup function called for the emulated hardware is dwmac1000_setup().
That function does not set the DMA rx or tx fifo size.

At the same time, the rx and tx fifo size is not provided in the
devicetree file (nuvoton-npcm750.dtsi), so the failure is obvious.

I understand that the real hardware may be based on a more recent
version of the DWMAC IP which provides the DMA tx/rx fifo size, but
I do wonder: Are the benefits of this patch so substantial that it
warrants breaking the qemu emulation of this network interface ?

Thanks,
Guenter