Re: [PATCH 1/1] net: dsa: rtl8366rb: standardize init jam tables

On Mon, Jan 25, 2021 at 7:00 AM Lorenzo Carletti
[off-list ref] wrote:

In the rtl8366rb driver there are some jam tables which contain
undocumented values.
While trying to understand what these tables actually do,
I noticed a discrepancy in how one of those was treated.
Most of them were plain u16 arrays, while the ethernet one was
an u16 matrix.
By looking at the vendor's droplets of source code these tables came from,
I found out that they were all originally u16 matrixes.

This commit standardizes the jam tables, turning them all into
u16 matrixes.
This change makes it easier to understand how the jam tables are used
and also makes it possible for a single function to handle all of them,
removing some duplicated code.

...

Since further replies removed code, I reply here for everybody with an
example of my thoughts against this cryptic data.

If you look into the below, for example, you may notice a few things.
 - it pokes different address regions (sounds like data section, text
section, etc.)
 - it has different meaning for some addresses (0xBE prefix)

+static const u16 rtl8366rb_init_jam_f5d8235[][2] = {
+       {0x0242, 0x02BF}, {0x0245, 0x02BF}, {0x0248, 0x02BF}, {0x024B, 0x02BF},
+       {0x024E, 0x02BF}, {0x0251, 0x02BF}, {0x0254, 0x0A3F}, {0x0256, 0x0A3F},
+       {0x0258, 0x0A3F}, {0x025A, 0x0A3F}, {0x025C, 0x0A3F}, {0x025E, 0x0A3F},

Sounds like we program some buffer lengths / limits (0x2c0, 0xa40 if
it rings any bell to anybody).

+       {0x0263, 0x007C}, {0x0100, 0x0004}, {0xBE5B, 0x3500}, {0x800E, 0x200F},

BE5B seems like "execute the routine at 0x3500 address".
Thus I think shuffling those pairs before 0xbe shouldn't give any
difference (but I have no hw to try).

+       {0xBE1D, 0x0F00}, {0x8001, 0x5011}, {0x800A, 0xA2F4}, {0x800B, 0x17A3},
+       {0xBE4B, 0x17A3}, {0xBE41, 0x5011}, {0xBE17, 0x2100}, {0x8000, 0x8304},
+       {0xBE40, 0x8304}, {0xBE4A, 0xA2F4}, {0x800C, 0xA8D5}, {0x8014, 0x5500},
+       {0x8015, 0x0004}, {0xBE4C, 0xA8D5}, {0xBE59, 0x0008}, {0xBE09, 0x0E00},
+       {0xBE36, 0x1036}, {0xBE37, 0x1036}, {0x800D, 0x00FF}, {0xBE4D, 0x00FF},

0x80 addresses are some kind of magic, like interrupt vector returns
or so. You may notice some 0xBE commands against the addresses that
are put into the 0x8000 address region.

 };

Overall it seems you have to discover a full firmware image to make
any assumptions about CPU ISA used there and address mapping.

-- 
With Best Regards,
Andy Shevchenko