RE: [PATCH] net: ti: netcp: restore get/set_pad_info() functionality

-----Original Message-----
From: David Laight [mailto:David.Laight@ACULAB.COM]
Sent: Tuesday, February 09, 2016 9:13 AM
To: Strashko, Grygorii; netdev@vger.kernel.org; David S . Miller; Arnd Bergmann
Cc: Cooper Jr., Franklin; Nori, Sekhar; linux-kernel@vger.kernel.org; Kwok, WingMan;
Karicheri, Muralidharan; N, Mugunthan V
Subject: RE: [PATCH] net: ti: netcp: restore get/set_pad_info() functionality

From: Grygorii Strashko

quoted

Sent: 09 February 2016 13:58
From: Arnd Bergmann <arnd@arndb.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:

quoted

 	void *buf_ptr;

...

quoted

+		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?

No. At the end of the descriptor, host software can add scratchpad which is
not modified by the hardware, but is used by the driver. So please don't
rename.

Murali

David