On 14/04/2023 14.32, David Laight wrote:
From: Song, Yoong Siang
Sent: 14 April 2023 12:16
...
I have checked Foxville manual for SRRCTL (Split and Replication Receive
Control) register and below GENMASKs looks correct.
-#define IGC_SRRCTL_BSIZEPKT_SHIFT 10 /* Shift _right_ */
-#define IGC_SRRCTL_BSIZEHDRSIZE_SHIFT 2 /* Shift _left_ */
+#define IGC_SRRCTL_BSIZEPKT_MASK GENMASK(6, 0)
+#define IGC_SRRCTL_BSIZEPKT_SHIFT 10 /* Shift _right_ */
Shift due to 1 KB resolution of BSIZEPKT (manual field BSIZEPACKET)
Ya, 1K = BIT(10), so need to shift right 10 bits.
I bet the code would be easier to read if it did 'value / 1024u'.
The object code will be (much) the same.
I agree. Code becomes more readable for humans and machine code will be
the same.
+#define IGC_SRRCTL_BSIZEHDRSIZE_MASK GENMASK(13, 8)
+#define IGC_SRRCTL_BSIZEHDRSIZE_SHIFT 2 /* Shift _left_ */
This shift is suspicious, but as you inherited it I guess it works.
I did the math, and it happens to work, knowing (from manual) value is in 64 bytes
resolution.
It is in 64 = BIT(6) resolution, so need to shift right 6 bits.
But it start on 8th bit, so need to shift left 8 bits.
Thus, total = shift left 2 bits.
I didnt put the explanation into the header file because it is too lengthy
and user can know from databook.
Well, users usually don't have access to the databook (Programming
Interface) PDF. Personally I have it, but I had to go though a lot of
red-tape to get it (under Red Hat NDA).
How do you feel on the necessary of explaining the shifting logic?
Not everyone trying to grok the code will have the manual.
Even writing (8 - 6) will help.
Or (I think) if the value is in bits 13-8 in units of 64 then just:
((value >> 8) & 0x1f) * 64
gcc will do a single shift right and a mask 9at some point).
You might want some defines, but if they aren't used much
just comments that refer to the names in the manual/datasheet
can be enough.
After Alexander Lobakin opened my eyes for GENMASK, FIELD_PREP and
FIELD_GET, I find that easier to read and work-with these kind of
register value manipulations, see[1] include/linux/bitfield.h. It will
also detect if the assigned value exceeds the mask (like David code
handled via mask). (thx Alex)
[1]
https://elixir.bootlin.com/linux/v6.3-rc6/source/include/linux/bitfield.h#L14
So, instead of:
srrctl |= IGC_RX_HDR_LEN << IGC_SRRCTL_BSIZEHDRSIZE_SHIFT;
I would write
/* BSIZEHDR value in 64 bytes resolution */
srrctl |= FIELD_PREP(IGC_SRRCTL_BSIZEHDRSIZE_MASK, (IGC_RX_HDR_LEN /
64));
--Jesper
