netdev - RE: [PATCH] net: ti: netcp: restore get/set_pad

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Date:	Tue, 9 Feb 2016 14:13:04 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Grygorii Strashko' <grygorii.strashko@...com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S . Miller" <davem@...emloft.net>,
	"Arnd Bergmann" <arnd@...db.de>
CC:	Franklin S Cooper Jr <fcooper@...com>,
	Sekhar Nori <nsekhar@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wingman Kwok <w-kwok2@...com>,
	Murali Karicheri <m-karicheri2@...com>,
	Mugunthan V N <mugunthanvnm@...com>
Subject: RE: [PATCH] net: ti: netcp: restore get/set_pad_info() functionality

From: Grygorii Strashko
> Sent: 09 February 2016 13:58
> From: Arnd Bergmann <arnd@...db.de>
> 
> The commit 899077791403 ("netcp: try to reduce type confusion in descriptors")
> introduces a regression in Kernel 4.5-rc1 and it breaks
> get/set_pad_info() functionality.
> 
> The TI NETCP driver uses pad0 and pad1 fields of knav_dma_desc to
> store DMA/MEM buffer pointer and buffer size respectively. And in both
> cases for Keystone 2 the pointer type size is 32 bit regardless of
> LAPE enabled or not, because CONFIG_ARCH_DMA_ADDR_T_64BIT originally
> is not expected to be defined.
> 
> 			!LAPE	LPAE
> sizeof(void*)		32bit	32bit
> sizeof(dma_addr_t) 	32bit	32bit
> sizeof(phys_addr_t) 	32bit	64bit
> 
> Unfortunately, above commit changed buffer's pointers save/restore
> code (get/set_pad_info()) and added intermediate conversation to u64
> which works incorrectly on 32bit Keystone 2 and causes TI NETCP driver
> crash in RX/TX path due to "Unable to handle kernel NULL pointer"
> exception. This issue was reported and discussed in [1].
> 
> Hence, fix it by partially reverting above commit and restoring
> get/set_pad_info() functionality as it was before.

You should really get rid of most of the horrid pointer-integer casts.
Code like:
>  	void *buf_ptr;
...
> +		get_pad_info((u32 *)&buf_ptr, &buf_len, ndesc);
is just asking for trouble.

You'd be better using assignments like:
	buf_ptr = (cast)get_pad_0(ndesc);
	buf_len = get_pad_1(ndesc);
Then the values are passed (and cast) as numerics.

In reality the 'pad' fields ought to be renamed - since they aren't pads.
Perhaps they should be a union?

	David