Re: [PATCH v10 3/4] fpga: dfl: add basic support for DFHv1
From: matthew . gerlach
Date: Fri Jan 13 2023 - 14:04:48 EST
On Fri, 13 Jan 2023, Xu Yilun wrote:
On 2023-01-12 at 07:36:29 -0800, matthew.gerlach@xxxxxxxxxxxxxxx wrote:
On Thu, 12 Jan 2023, Andy Shevchenko wrote:
On Wed, Jan 11, 2023 at 10:13:31AM +0800, Xu Yilun wrote:
On 2023-01-10 at 14:07:16 -0800, matthew.gerlach@xxxxxxxxxxxxxxx wrote:
On Tue, 10 Jan 2023, Andy Shevchenko wrote:
On Mon, Jan 09, 2023 at 04:30:28PM -0800, matthew.gerlach@xxxxxxxxxxxxxxx wrote:
From: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
...
v10: change dfh_find_param to return size of parameter data in bytes
The problem that might occur with this approach is byte ordering.
When we have u64 items, we know that they all are placed in CPU
ordering by the bottom layer. What's the contract now? Can it be
a problematic? Please double check this (always keep in mind BE32
as most interesting case for u64/unsigned long representation and
other possible byte ordering outcomes).
A number of u64 items certainly states explicit alignment of the memory, but
I think byte ordering is a different issue.
The bottom layer, by design, is still enforcing a number u64 items under the
hood. So the contract has not changed. Changing units of size from u64s to
bytes was suggested to match the general practice of size of memory being in
bytes. I think the suggestion was made because the return type for
dfh_find_param() changed from u64* to void* in version 9, when indirectly
returning the size of the parameter data was introduced. So a void * with a
size in bytes makes sense. On the other hand, returning a u64 * is a more
precise reflection of the data alignment. I think the API should be as
I prefer (void *) + bytes. The properties in the parameter block are not
guarateed to be u64 for each, e.g. the REG_LAYOUT, so (void *) could better
indicate it is not. It is just a block of data unknown to DFL core and to
be parsed by drivers.
If the hardware / protocol is capable of communicating the arbitrary lengths
of parameters, then yes, bytes make sense. But this should be clear what byte
ordering is there if the items can be words / dwords / qwords.
The hardware does communicate the arbitrary lengths of the parameter data;
so bytes make sense. I will update Documentation/fpga/dfl.rst to explicitly
say that multi-byte quantities are little-endian.
TL;DR: The Q is: Is the parameter block a byte stream? If yes, then your
proposal is okay. If no, no void * should be used. In the latter it should
be union of possible items or a like as defined by a protocol.
The parameter block is not a byte stream; so void * should be used.
Mm.. I think Andy's idea is, if the parameter block is not a byte stream,
void * should NOT be used.
My understanding is, The parameter block is not a byte stream in HW, it is
some items (or properties) of various lengths. They are compacted in the
parameter block. But the layout is not generally defined, each parameter
block could have its own layout.
Your understanding is correct that the parameter block is a set of items
(or properties) of variouse lengths in HW. The parameter blocks are
comparable to PCI capabilities in PCI config space. Each capability has its own
defined stucture.
The definition and layout of the parameter block is specific to each device,
that is, people design the parameter block for the device when they design
the device. So DFL core doesn't try to generalize all the layouts, they
are unlimited. DFL core just see it as a block of untouched data to be parsed
by each driver. So from DFL core's perspective, it is a byte stream.
Yes, from the DFL core's perspective, the parameter blocks are opaque
chunks of data. This would affirm your preference of using (void *) and
byte size in the API for the function, dfh_find_param.
Thanks,
Matthew Gerlach
Thanks,
Yilun
Thanks,
Matthew Gerlach
And why users/drivers need to care about the alignment of the parameter
block?
follows:
--
With Best Regards,
Andy Shevchenko