On 13/07/2020 14:07, Cristian Marussi wrote:
Hi Steven
thanks for the review.
On Mon, Jul 13, 2020 at 12:20:43PM +0100, Steven Price wrote:
On 10/07/2020 14:39, Cristian Marussi wrote:
Remove __packed attribute from struct scmi_event_header.
Signed-off-by: Cristian Marussi <cristian.maru...@arm.com>
A drive-by review. But this doesn't look safe to me. sizeof(struct
scmi_event_header) is used in several places and this change will modify
that from 13 to 16, but leave the structure members at the same offset (as
the members are naturally aligned). In particular the na�ve header size is
now bigger than the offset to payld.
What is the justification for __packed being 'unneeded'?
Arnd pointed out at first that this structure in the original series had a mix
of
fixed and non-fixed types in its fields and that the __packed rendered some
fields
misaglined.
Removing that as it is, in fact left also some unexplained implicit padding
which is
at odd for a struct containing fixed-sized types.
In a following fix in the series I have indeed moved this struct's fields and
others
to generic non fixed type fields and shuffled around the fields to avoid
misalignment
and implicit internal padding (except for the trailing padding due to the
variable
size array)
It was probably better to squash also this patch in that following patch.
This structure is used internally to push variable-sized (through the means of
the payld[])
events descriptors through a fifo from the ISR to the deferred workqueus, so
that's whhy I
originally thought to avoid to carry around unneeded padding into the fifos and
use the
__packed.
On the correctness side, as you pointed out, the header with padding is now 16
so when
I push thorugh the kfifos this header and the payload there's a hole in the
data as
represented in the fifo buffer as such
@end_of hdr+payld kfifo writes:
kifo_in(fifo, h, sizeof(*h)) + kfifo_in(fifo, payld, h->payld_sz)
0 14 16
+-------+----+------------
|header - pad| payload...
-------------------------
^
|
.payld
(Note that header and payload comes from two distinct place so I have push it
with two kfifo_in()
in order to avoid a redundant memcpy on an intermediate buffer to collate
them...thing
that was pointed out as undesirable in a review)
and when I read it back from the fifo such hole is just transparently
overwritten:
@header read:
kfifo_out(fifo, h, sizeof(*h))
0 14 16
+-------+----+--------------
|header - pad|
----------------------------
@payload_read:
kfifo_out(fifo, h->payld, h->payld_sz)
0 14
+-------+----+--------------
|header | payload....
----------------------------
^
|
.payld
So since anyway the drawback of packing is that the misaglined access
potentially slows down the
reads, I was not sure anymore it was worth to pack and misalign, and, given
that it seemed not
to be liked so much, I dropped it and moved to generic non-fixed types without
packing.
Thanks for the explanation - it sounds like the change is correct.
However, from the description above it sounds like splitting the header
and payload into separate types would be clearer. I'm not sure the
flexible length array is adding to code clarity here. In particular the
'pad' being put into the fifo is actually going to be a (truncated) copy
of the payload.
There is also a trick with an unnamed internal struct which gets the
padding in the correct place...
struct scmi_event_header {
struct {
u64 timestamp;
u8 evt_id;
size_t payld_sz;
}
u8 payld[];
};
...with that then...
offsetof(struct scmi_event_header, payld) ==
sizeof(struct scmi_event_header)
...which avoids the need for kfifo_out having to overwrite the padding.
A better (and shorter) explanation of all of the above is possibly needed (but
I'd still prefer
the fixed sized typing and __packed 'holeless' approach...)
Fixed sized types and __packed is easier to reason about, but obviously
naturally aligned types do tend to be faster.
Steve
