netdev - Re: [PATCH 2/2] net: moxa: prevent double-mapping of DMA areas

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Date:   Thu, 18 Aug 2022 21:27:05 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Sergei Antonov <saproj@...il.com>
Cc:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: [PATCH 2/2] net: moxa: prevent double-mapping of DMA areas

On Thu, Aug 18, 2022 at 09:29:48PM +0300, Sergei Antonov wrote:
> Fix the warning poping up after bringing the link down, then up:
>  ip link set dev eth0 down
>  ip link set dev eth0 up
> 
> WARNING: CPU: 0 PID: 55 at kernel/dma/debug.c:570 add_dma_entry+0x204/0x2ec
> DMA-API: moxart-ethernet 92000000.mac: cacheline tracking EEXIST, overlapping mappings aren't supported
> CPU: 0 PID: 55 Comm: ip Not tainted 5.19.0+ #57
> Hardware name: Generic DT based system
>  unwind_backtrace from show_stack+0x10/0x14
>  show_stack from dump_stack_lvl+0x34/0x44
>  dump_stack_lvl from __warn+0xbc/0x1f0
>  __warn from warn_slowpath_fmt+0x94/0xc8
>  warn_slowpath_fmt from add_dma_entry+0x204/0x2ec
>  add_dma_entry from dma_map_page_attrs+0x110/0x328
>  dma_map_page_attrs from moxart_mac_open+0x134/0x320
>  moxart_mac_open from __dev_open+0x11c/0x1ec
>  __dev_open from __dev_change_flags+0x194/0x22c
>  __dev_change_flags from dev_change_flags+0x14/0x44
>  dev_change_flags from devinet_ioctl+0x6d4/0x93c
>  devinet_ioctl from inet_ioctl+0x1ac/0x25c
> 
> Unmap RX memory areas in moxart_mac_stop(), so that moxart_mac_open()
> will map them anew instead of double-mapping. To avoid code duplication,
> create a new function moxart_mac_unmap_rx(). Nullify unmapped pointers to
> prevent double-unmapping (ex: moxart_mac_stop(), then moxart_remove()).

This makes the code symmetric, which is good.

However, moxart_mac_free_memory() will also free the descriptors,
which is not required. moxart_remove() should undo what the probe did,
nothing more.

	Andrew