Re: [net,1/1] net: stmmac: Set OWN bit last in dwmac4_set_rx_owner()

[Andrew Lunn]

On Fri, Aug 09, 2024 at 10:42:29PM +0800, Tan En De wrote:
> Ensure that all other bits in the RDES3 descriptor are configured before
> transferring ownership of the descriptor to DMA via the OWN bit.

Are you seeing things going wrong with real hardware, or is this just
code review? If this is a real problem, please add a description of
what the user would see.

Does this need to be backported in stable?

https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html

If it does, you should add a Fixes: tag and 'Cc: stable@xxxxxxxxxxxxxxx'

This will also decide which tree you need to base the patch on:

https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html#netdev-faq

>  static void dwmac4_set_rx_owner(struct dma_desc *p, int disable_rx_ic)
>  {
> -	p->des3 |= cpu_to_le32(RDES3_OWN | RDES3_BUFFER1_VALID_ADDR);
> +	p->des3 |= cpu_to_le32(RDES3_BUFFER1_VALID_ADDR);
>  
>  	if (!disable_rx_ic)
>  		p->des3 |= cpu_to_le32(RDES3_INT_ON_COMPLETION_EN);
> +
> +	dma_wmb();
> +	p->des3 |= cpu_to_le32(RDES3_OWN);

Is the problem here that RDES3_INT_ON_COMPLETION_EN is added after the
RDES3_OWN above has hit the hardware, so it gets ignored?

It seems like it would be better to calculate the value in a local
variable, and then assign to p->des3 once.

	  Andrew