netdev - Re: [PATCH V4 net 3/5] net: stmmac: fix dma physical address of descriptor when display ring

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Message-ID: <20210206122911.5037db4c@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Sat, 6 Feb 2021 12:29:11 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Joakim Zhang <qiangqing.zhang@....com>
Cc:     peppe.cavallaro@...com, alexandre.torgue@...com,
        joabreu@...opsys.com, davem@...emloft.net, netdev@...r.kernel.org,
        linux-imx@....com
Subject: Re: [PATCH V4 net 3/5] net: stmmac: fix dma physical address of
 descriptor when display ring

On Thu,  4 Feb 2021 19:21:42 +0800 Joakim Zhang wrote:
> Driver uses dma_alloc_coherent to allocate dma memory for descriptors,
> dma_alloc_coherent will return both the virtual address and physical
> address. AFAIK, virt_to_phys could not convert virtual address to
> physical address, for which memory is allocated by dma_alloc_coherent.
> 
> Signed-off-by: Joakim Zhang <qiangqing.zhang@....com>

What does this patch fix? Theoretically incorrect value in a debug dump
or are you actually observing incorrect behavior?

> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c b/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> index c6540b003b43..6f951adc5f90 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> @@ -402,7 +402,8 @@ static void dwmac4_rd_set_tx_ic(struct dma_desc *p)
>  	p->des2 |= cpu_to_le32(TDES2_INTERRUPT_ON_COMPLETION);
>  }
>  
> -static void dwmac4_display_ring(void *head, unsigned int size, bool rx)
> +static void dwmac4_display_ring(void *head, unsigned int size, bool rx,
> +				dma_addr_t dma_rx_phy, unsigned int desc_size)
>  {
>  	struct dma_desc *p = (struct dma_desc *)head;
>  	int i;
> @@ -410,8 +411,8 @@ static void dwmac4_display_ring(void *head, unsigned int size, bool rx)
>  	pr_info("%s descriptor ring:\n", rx ? "RX" : "TX");
>  
>  	for (i = 0; i < size; i++) {
> -		pr_info("%03d [0x%x]: 0x%x 0x%x 0x%x 0x%x\n",
> -			i, (unsigned int)virt_to_phys(p),
> +		pr_info("%03d [0x%llx]: 0x%x 0x%x 0x%x 0x%x\n",
> +			i, (unsigned long long)(dma_rx_phy + i * desc_size),
>  			le32_to_cpu(p->des0), le32_to_cpu(p->des1),
>  			le32_to_cpu(p->des2), le32_to_cpu(p->des3));
>  		p++;

Why do you pass the desc_size in? The virt memory pointer is incremented
by sizeof(*p) surely

	dma_addr + i * sizeof(*p) 

would work correctly? Also please use the correct print format for
dma_addr_t, you shouldn't have to cast.