netdev - Re: [PATCH net 1/2] net: stmmac: Limit FIFO size by hardware feature value

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Message-ID: <c0fc9c74-1ea6-463a-85c1-be9152c41009@socionext.com>
Date: Tue, 21 Jan 2025 09:17:18 +0900
From: Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
To: Andrew Lunn <andrew@...n.ch>
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>, netdev@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/2] net: stmmac: Limit FIFO size by hardware feature
 value

Hi Andrew,

On 2025/01/21 1:29, Andrew Lunn wrote:
> On Mon, Jan 20, 2025 at 02:20:23PM +0900, Kunihiko Hayashi wrote:
>> Hi Andrew,
>>
>> On 2025/01/17 5:16, Andrew Lunn wrote:
>>> On Thu, Jan 16, 2025 at 11:08:52AM +0900, Kunihiko Hayashi wrote:
>>>> Tx/Rx FIFO size is specified by the parameter "{tx,rx}-fifo-depth"
> from
>>>> the platform layer.
>>>>
>>>> However, these values are constrained by upper limits determined by
> the
>>>> capabilities of each hardware feature. There is a risk that the
> upper
>>>> bits will be truncated due to the calculation, so it's appropriate
> to
>>>> limit them to the upper limit values.
>>>
>>> Are these values hard coded in the platform layer? Or can they come
>>> from userspace?
>>
>> My explanation is insufficient and misleading.
>> "From the platform layer" means the common layer of stmmac described in
>> "stmmac_platform.c".
>>
>>> If they are hard coded, we should also fix them. So maybe add a
>>> netdev_warn(), and encourage the platform maintainers to fix their
>>> platform. If they are coming from userspace, we should consider
>>> failing the ethtool call with an -EINVAL, and maybe an extack with the
>>> valid range?
>>
>> These values are derived from the devicetree and stored in the stmmac
>> private structure. They are hardware-specific values, so I think this
>> fix is sufficient.
> 
> But if they are coming from device tree, the device tree developer has
> made an error, which has been silently ignored. Do we want to leave
> the device tree broken? Or should we encourage developers to fix them?
> Printing a warning would facilitate that.

I think that developers should fix the devicetree, so I'll add a warning
message if the specified value exceeds the hardware capability.

Thank you,

---
Best Regards
Kunihiko Hayashi